Analog Inputs - ADS1115¶
Overview¶
The 4 channel ADS1115 DAC with op-amp buffering provides high-impedance, accurate measurements of Battery Voltage, Alternator Current, Engine Speed, and Thermistor Temp with negligible power consumption. The design is hardened for noisy marine environments.
Design Considerations¶
Op-Amp Output Voltage Constraints¶
Why Op-Amp Buffering is Required: The high-impedance voltage dividers (typically 384kΩ to 1MΩ+) used to minimize current draw cannot directly drive the ADS1115 input without significant loading errors, especially if protection diodes are used. Op-amp buffers provide the necessary impedance transformation from high-impedance sources to low-impedance ADC inputs while at the same time constaining inputs to the safe range.
However, this introduces an imporant limitation: Even with the excellent rail-to-rail TLV9154IDR op-amp, the minimum output voltage is approximately 10-20mV (typical VOL specification). This directly limits the minimum measurable input values on all 4 ADS channels:
| Channel | Minimum Op-Amp Output | Minimum Measurable Input | Impact |
|---|---|---|---|
| 0 (Battery) | ~10mV | 0.21V | No practical impact (batteries never this low) |
| 1 (Current) | ~10mV | -246A | No impact (within ±200A sensor range) |
| 2 (RPM) | ~10mV | 8Hz → 27-479 RPM | min idle detection depends on signal freq |
| 3 (Temperature) | ~10mV | Depends on configuration | Limits cold temperature range |
ADS1115 Input Range Limitations¶
Specification Clarity: The ADS1115 specifications are easy to misunderstand:
- Advertised range: ±4.096V (with GAIN = 1 setting)
- Reality: Practical range for this use case is 0V to +3.3V maximum due to single 3.3V supply rail
- Negative voltages: Cannot be measured - ADS1115 inputs must remain positive
These constraints must be considered for all channels.
System Architecture¶
Signal Chain Architecture¶
All analog input channels share a common architecture*:
Input Signal ──[R1]──┬──[R2]──GND
│
[5nF]──GND (Low-pass filter)
│
[TLV9154IDR Input+]
│
[TLV9154IDR Output]──ADS1115 Channel (0-3.3V only)
│
[Feedback to Input-] (Unity gain)
Component Functions¶
Voltage Divider (R1/R2)¶
- Function: Scales high input voltages to ADS1115-compatible range (0-3.3V maximum)
- Power consumption: Continuous, proportional to input voltage
- Ratio selection: Determines maximum measurable voltage and resolution
- Precision: ±0.1% tolerance for accuracy
Low-Pass Filter (5nF Capacitor)¶
- Cutoff frequency: f = 1 / (2π × R_thevenin × 5nF)
- Function: Removes high-frequency noise and EMI before amplification
- Settling time: ~5 × time constant for 99% accuracy
- Placement: Before op-amp buffer for optimal noise rejection
Unity-Gain Buffer (TLV9154IDR)¶
- Function: High input impedance to low output impedance conversion
- Gain: Exactly 1.0 (Vout = Vin)
- Supply: Single 3.3V rail
- Configuration: Non-inverting input from voltage divider, output feedback to inverting input
- Input impedance: >1MΩ (isolates high-impedance voltage dividers)
- Critical limitation: Minimum output ~10-20mV limits low-signal detection
ADC (ADS1115)¶
- Resolution: 16-bit with 0 to +3.3V input range (not ±4.096V as commonly stated)
- Interface: I2C communication
- LSB resolution: 0.1mV per count (3.3V ÷ 32768 counts)
Single-Supply Design Benefits¶
Benefits: - Inherent overvoltage protection: Op-amp output cannot exceed 3.3V supply rails - No protection diodes needed: Eliminates leakage current and temperature coefficient errors - Perfect accuracy: No diode-related drift or offset issues - Low power consumption: Single 3.3V supply shared with ADS1115
Channel Specifications¶
Channel 0: Battery Voltage Monitor¶
Application: battery voltage monitoring for 12V, 24V, and 48V
Design Parameters¶
- R1: 1MΩ ±0.1%, 1/8W, 0805 SMD
- R2: 49.9kΩ ±0.1%, 1/8W, 0805 SMD
- Divider ratio: 0.0475 (1:21.04 scaling)
- Filter capacitor: 5nF ±10%, X7R, 0603 SMD
- Measurement range: 0.21V to 65.2V
- Minimum measurable: 0.21V (limited by op-amp minimum output)
Performance Analysis¶
| Input Voltage | Divided Voltage | ADC Reading | ADC Resolution (LSB) | Engineering Resolution | Practical Accuracy |
|---|---|---|---|---|---|
| 0.21V | 0.010V | Op-amp minimum | 0.1mV | 0.0021V | Limited by noise/drift |
| 2.1V | 0.100V | Valid | 0.1mV | 0.0021V | 2.6% |
| 5.0V | 0.238V | Valid | 0.1mV | 0.0021V | 1.1% |
| 12.0V | 0.570V | Valid | 0.1mV | 0.0021V | 0.46% |
| 24.0V | 1.141V | Valid | 0.1mV | 0.0021V | 0.23% |
| 48.0V | 2.281V | Valid | 0.1mV | 0.0021V | 0.11% |
| 60.0V | 2.852V | Valid | 0.1mV | 0.0021V | 0.092% |
| 65.2V | 3.100V | Valid | 0.1mV | 0.0021V | 0.084% |
Resolution:
- ADC LSB resolution: 0.1mV (3.3V ÷ 32768 counts)
- Engineering resolution: 0.0021V input (0.1mV ÷ 0.0475 divider ratio)
- Practical accuracy: Limited by component tolerances (±0.1% resistors), temperature drift, and noise floor
- Effective resolution: ~55mV due to noise floor and component limitations
Filter Characteristics¶
- Thevenin resistance: 47.5kΩ
- Cutoff frequency: 670Hz
- Time constant: 0.24ms
- 99% settling time: 1.2ms
Power Consumption Analysis¶
| Input Voltage | Divider Current | Divider Power | Total Power | Equivalent at 12V |
|---|---|---|---|---|
| 12.0V | 11.4µA | 0.14mW | 0.27mW | 22.5µA |
| 24.0V | 22.9µA | 0.55mW | 0.68mW | 56.7µA |
| 48.0V | 45.7µA | 2.19mW | 2.32mW | 193µA |
| 60.0V | 57.1µA | 3.43mW | 3.56mW | 297µA |
Channel 1: Alternator Current Monitor¶
Application: QNHC1K-21 200A Hall Effect Current Sensor monitoring (2.5V @ 0A, ±2V swing for ±200A)
Design Parameters¶
- R1: 768kΩ ±0.1%, 1/8W, 0805 SMD
- R2: 768kΩ ±0.1%, 1/8W, 0805 SMD
- Divider ratio: 0.5000 (1:2 scaling)
- Filter capacitor: 5nF ±10%, X7R, 0603 SMD
- Sensor voltage range: 0.5V to 4.5V (±200A)
- ADC voltage range: 0.25V to 2.25V
- Minimum measurable: 0.02V sensor voltage (limited by op-amp, equivalent to -248A but real limit is -200 (sensor))
Performance Analysis¶
| Hall Sensor Voltage | Current (A) | Divided Voltage | ADC Reading | ADC Resolution (LSB) | True Current Resolution | Practical Accuracy | Measurable |
|---|---|---|---|---|---|---|---|
| 0.5V | -200A | 0.250V | Valid | 0.1mV | 0.05A | ±1.04A | ✅ Valid |
| 2.5V | 0A | 1.250V | Valid | 0.1mV | 0.05A | ±1.04A | ✅ Valid |
| 4.5V | +200A | 2.250V | Valid | 0.1mV | 0.05A | ±1.04A | ✅ Valid |
Resolution: - ADC LSB resolution: 0.1mV (3.3V ÷ 32768 counts) - Sensor scaling: 100A/V (200A range ÷ 2V swing) - True current resolution: 0.1mV ÷ 0.5 divider × 100A/V = 0.02A per LSB - Practical accuracy: ±1.04A represents the noise floor and worst-case measurement uncertainty, not the fundamental resolution - Noise sources: Op-amp noise, ADC noise, sensor drift, temperature effects, EMI - Effective resolution: While theoretical resolution is 0.02A, practical measurements are limited to ~1A accuracy due to system noise and sensor specifications
Note: Op-amp minimum output (10mV) corresponds to -246A, well outside the ±200A physical sensor limits, so this has no practical impact.
Filter Characteristics¶
- Thevenin resistance: 384kΩ
- Cutoff frequency: 83Hz
- Time constant: 1.9ms
- 99% settling time: 9.6ms
Power Consumption Analysis¶
| Input Voltage | Divider Current | Divider Power | Total Power | Equivalent at 12V |
|---|---|---|---|---|
| 2.5V | 1.63µA | 4.07µW | 0.13mW | 10.8µA |
| 4.0V | 2.60µA | 10.4µW | 0.14mW | 11.7µA |
Channel 2: Engine Speed Monitor¶
Application: LM2907 frequency-to-voltage converter output from alternator stator tap
Design Parameters¶
- Direct connection: LM2907 output directly to op-amp input (no voltage divider)
- Filter capacitor: 5nF ±10%, X7R, 0603 SMD
- LM2907 output range: 0V to 5V
- Op-amp output range: 0V to 3.3V (clipped by supply rail)
- ADC voltage range: 0V to 3.3V
- Minimum measurable: 0.01V LM2907 output (8Hz minimum frequency)
- Maximum measurable: 3.3V LM2907 output (2640Hz maximum frequency)
LM2907 Circuit Configuration¶
Power & Bypass
- Pin 6 (V+): 5V supply
- Pin 5 (COL): 5V supply
- Pin 8 (GND): System ground
- Bypass capacitor: 1µF from Pin 6 to ground
Timing Components (Frequency-to-Voltage Conversion) - Pin 2 (CP1): 10nF capacitor to ground - Pin 3 (CP2/IN+): - Three 1µF capacitors to ground (total 3µF) - 25kΩ resistor to ground - Transfer function: VO = VCC × fIN × C1 × R1 = 5V × fIN × 10nF × 25kΩ - Scaling factor: VO = fIN × 0.00125 V/Hz
Output Configuration - Pin 4 (EMIT): Connected to Pin 7 and 10kΩ pull-down, feeds voltage divider - Pin 7 (IN−): Connected to Pin 4 (feedback)
Input Signal Conditioning (Pin 1 – TACH+) - AC coupling: Two 10µF/100V capacitors in parallel (20µF total) - Series resistance: 4.7kΩ, 2W (HP122WJ0472T4E) - Input filtering: 6.8nF capacitor to ground (5kHz cutoff) - Input termination: 100kΩ resistor to ground - Overvoltage protection: SMBJ12CA bidirectional TVS diode (12V clamp) - High-pass cutoff: 1/(2π × 100kΩ × 20µF) = 0.08Hz - Voltage attenuation: ~4.5% (4.7kΩ + 100kΩ divider)
Frequency and RPM Analysis¶
Engine-to-Stator Scaling Factors:
- Conservative case: 6-pulse alternator, 1.5:1 belt ratio → Engine RPM × 0.15 = Stator Hz
- Aggressive case: 7-pulse alternator, 2.5:1 belt ratio → Engine RPM × 0.292 = Stator Hz
- Worst case: 1-pulse per revolution, 1:1 direct → Engine RPM × 0.0167 = Stator Hz
Performance Analysis (Conservative Case Example)¶
Minimum detectable frequency: 8Hz Maximum detectable frequency: 2640Hz (limited by 3.3V op-amp supply rail)
| Engine RPM | Stator Frequency | LM2907 Output | ADC Voltage | Status |
|---|---|---|---|---|
| 53 RPM | 8Hz | 0.010V | 0.010V | Minimum detectable |
| 133 RPM | 20Hz | 0.025V | 0.025V | Valid |
| 300 RPM | 45Hz | 0.056V | 0.056V | Valid |
| 600 RPM | 90Hz | 0.113V | 0.113V | Valid |
| 1500 RPM | 225Hz | 0.281V | 0.281V | Valid |
| 3000 RPM | 450Hz | 0.563V | 0.563V | Valid |
| 4000 RPM | 600Hz | 0.750V | 0.750V | Valid |
| 6000 RPM | 900Hz | 1.125V | 1.125V | Valid |
| 8000 RPM | 1200Hz | 1.500V | 1.500V | Valid |
| 10667 RPM | 1600Hz | 2.000V | 2.000V | Valid |
| 14667 RPM | 2200Hz | 2.750V | 2.750V | Valid |
| 17600 RPM | 2640Hz | 3.300V | 3.300V | Maximum detectable |
RPM Range Summary (Significantly Improved):
- Conservative (6-pulse, 1.5:1): 53 RPM to 17,600 RPM
- Aggressive (7-pulse, 2.5:1): 27 RPM to 9,041 RPM
- 1-pulse direct: 479 RPM to 158,400 RPM
Input Signal Requirements¶
Minimum detectable signal: ±40mV at Pin 1 (LM2907 worst-case threshold *** datasheet unclear on this!!)
Input signal analysis for 100mV (±100mV) input:
| Frequency | Signal at Pin 1 | Detection | Engine RPM (Conservative) | Engine RPM (1-pulse) |
|---|---|---|---|---|
| 50Hz | 95.4mV | ✅ Valid | 333 RPM | 3000 RPM |
| 100Hz | 95.4mV | ✅ Valid | 667 RPM | 6000 RPM |
| 500Hz | 94.6mV | ✅ Valid | 3333 RPM | 30000 RPM |
| 800Hz | 93.0mV | ✅ Valid | 5333 RPM | 48000 RPM |
| 1000Hz | 91.8mV | ✅ Valid | 6667 RPM | 60000 RPM |
Very Low Frequency Analysis (1-pulse systems):
Minimum detectable: 8Hz = 479 RPM for 1-pulse systems
| Engine RPM | Frequency | Signal at Pin 1 | Detection Status |
|---|---|---|---|
| 479 RPM | 8Hz | Variable | Minimum detectable |
| 1500 RPM | 25Hz | 238mV | ✅ Valid |
| 3000 RPM | 50Hz | 477mV | ✅ Valid |
| 6000 RPM | 100Hz | 954mV | ✅ Valid |
| 18000 RPM | 300Hz | 286mV | ✅ Valid |
| 30000 RPM | 500Hz | 477mV | ✅ Valid |
| 60000 RPM | 1000Hz | 918mV | ✅ Valid |
| 95040 RPM | 1584Hz | 1510mV | ✅ Valid |
| 158400 RPM | 2640Hz | 2518mV | Maximum detectable |
Sine vs. Square Wave Performance: Both waveform types perform identically for frequency detection. The LM2907 detects zero crossings regardless of waveform shape, and AC coupling affects both equally at very low frequencies.
Normal AC Operation (3V to 58V inputs)¶
- No damage risk: AC signals don't cause sustained power dissipation
- TVS clipping: Signals above 12V get clipped to 12V, maintaining proper LM2907 operation
- Circuit robustness: Safe operation with input signals up to 58V+ amplitude
DC Fault Protection Analysis¶
Sustained DC voltage analysis (engine-off or fault conditions):
| DC Input | Current Through 4.7kΩ | Power in 4.7kΩ | Total Power | Equivalent at 12V | Status |
|---|---|---|---|---|---|
| 14V | 426µA | 0.85mW | 7.4mW | 0.62mA | ✅ Safe |
| 28V | 3.4mA | 54.5mW | 96.7mW | 8.1mA | ✅ Safe |
| 56V | 9.36mA | 412mW | 525mW | 43.8mA | ✅ Safe |
Component ratings: 4.7kΩ resistor (HP122WJ0472T4E) rated for 2W, providing excellent safety margin up to 56V DC faults.
Filter Characteristics (Optimized - No Voltage Divider)¶
- Input impedance: Direct connection to op-amp (>1MΩ input impedance)
- Cutoff frequency: Determined by LM2907 output impedance and 5nF capacitor
- Time constant: Minimal (direct connection)
- 99% settling time: <50µs
Power Consumption Analysis (No Voltage Divider)¶
- Voltage divider power: 0W (no divider resistors)
- Op-amp power: 0.13mW (shared IC allocation)
- Total power: 0.13mW
- Equivalent at 12V: 10.8µA (unchanged from IC power)
Channel 3: Temperature Monitor (5V Supply Design)¶
Application: Engine or ambient temperature monitoring using 10kΩ NTC thermistor
Current Design Parameters¶
- Supply voltage: 5V
- R1: 10kΩ ±0.1%, 1/8W, 0805 SMD (pullup to 5V)
- R2: 10kΩ NTC thermistor (user-supplied sensor, to ground)
- Divider configuration: 10kΩ pullup to 5V, thermistor to ground
- Filter capacitor: 5nF ±10%, X7R, 0603 SMD
Design Limitations with 5V Supply¶
Critical Issue: 5V supply with 10kΩ/10kΩ divider creates voltages above 3.3V for cold temperatures: - ADC cannot measure voltages above 3.3V (supply rail limitation) - Cold temperatures below ~15°C are unmeasurable - Temperature range: Limited to ~15°C to +125°C (cold temperature limitation) - ADC voltage range: 1.67V to 4.17V (but clipped at 3.3V maximum)
Thermistor Performance Analysis (5V Supply)¶
Voltage divider equation: V_out = 5V × R_thermistor / (10kΩ + R_thermistor)
| Temperature | Thermistor Resistance | Divider Voltage | ADC Reading | Measurable | Temperature Resolution |
|---|---|---|---|---|---|
| -40°C | ~195kΩ | 4.76V | 3.3V clipped | ❌ Unmeasurable | N/A |
| -20°C | ~84kΩ | 4.47V | 3.3V clipped | ❌ Unmeasurable | N/A |
| 0°C | ~27kΩ | 3.65V | 3.3V clipped | ❌ Unmeasurable | N/A |
| 15°C | ~15kΩ | 3.00V | 3.00V | ✅ Valid | ~1.2°C |
| 25°C | ~10kΩ | 2.50V | 2.50V | ✅ Valid | ~0.8°C |
| 50°C | ~3.9kΩ | 1.41V | 1.41V | ✅ Valid | ~1.0°C |
| 75°C | ~1.8kΩ | 0.76V | 0.76V | ✅ Valid | ~1.8°C |
| 100°C | ~0.9kΩ | 0.41V | 0.41V | ✅ Valid | ~3.2°C |
| 125°C | ~0.5kΩ | 0.24V | 0.24V | ✅ Valid | ~5.8°C |
Alternative Optimized Configuration (Recommendation)¶
For full temperature range capability: - Supply voltage: 3.3V (matches ADC supply, but was impossible on this iteration of board hardware) - R1: 15kΩ ±0.1%, 1/8W, 0805 SMD (pullup to 3.3V) - Temperature range: -40°C to +125°C (full range) - Power reduction: ~40% lower consumption
Original Configuration Limitations¶
The documented 5V/10kΩ configuration suffers from: - Cold temperature limitation: Voltages above 3.3V cannot be measured by ADC - Wasted voltage range: High voltages at cold temperatures exceed ADC capability - Higher power consumption: 5V supply slightly increases current draw
Filter Characteristics (5V Supply Design)¶
- Thevenin resistance: 5kΩ (parallel combination at mid-range)
- Cutoff frequency: 6.4kHz
- Time constant: 25µs
- 99% settling time: 125µs
Power Consumption Analysis (5V Supply Design)¶
| Temperature | Thermistor R | Divider Current | Divider Power | Total Power | Equiv at 12V |
|---|---|---|---|---|---|
| 15°C | 15kΩ | 200µA | 1.00mW | 1.13mW | 94.2µA |
| 25°C | 10kΩ | 250µA | 1.25mW | 1.38mW | 115µA |
| 100°C | 0.9kΩ | 459µA | 2.30mW | 2.43mW | 203µA |
| 125°C | 0.5kΩ | 476µA | 2.38mW | 2.51mW | 209µA |
IC Configuration Details¶
TLV9154IDR Quad Op-Amp Pinout (SOIC-14 Package)¶
| Pin | Symbol | Function | Connection |
|---|---|---|---|
| 1 | OUT1 | Op Amp A Output | ADS1115 Channel 0 (AIN0) |
| 2 | IN1- | Op Amp A Inverting Input | Connect to Pin 1 |
| 3 | IN1+ | Op Amp A Non-inverting Input | Channel 0 voltage divider output |
| 4 | V+ | Positive Power Supply | +3.3V |
| 5 | IN2+ | Op Amp B Non-inverting Input | Channel 1 voltage divider output |
| 6 | IN2- | Op Amp B Inverting Input | Connect to Pin 7 |
| 7 | OUT2 | Op Amp B Output | ADS1115 Channel 1 (AIN1) |
| 8 | OUT3 | Op Amp C Output | ADS1115 Channel 2 (AIN2) |
| 9 | IN3- | Op Amp C Inverting Input | Connect to Pin 8 |
| 10 | IN3+ | Op Amp C Non-inverting Input | LM2907_OUT DIRECT (NO DIVIDER) |
| 11 | V- | Negative Power Supply | GND (0V) |
| 12 | IN4+ | Op Amp D Non-inverting Input | Channel 3 voltage divider output |
| 13 | IN4- | Op Amp D Inverting Input | Connect to Pin 14 |
| 14 | OUT4 | Op Amp D Output | ADS1115 Channel 3 (AIN3) |
Unity Gain Configuration: Each op-amp output connects to its inverting input for 1:1 voltage following.
ADS1115 Configuration¶
Resolution - Theoretical ADC range: ±4.096V (GAIN = 1) - Actual usable range: 0V to +3.3V - 16-bit resolution: 0.1mV per LSB (3.3V ÷ 32768 counts) - Effective resolution: 0.1mV / divider_ratio per input LSB
System Performance Analysis¶
Total System Power Consumption¶
Per-Channel Power Analysis (Typical Operating Conditions)¶
| Channel | Application | Input Condition | Channel Power | IC Power Share | Total Power | Equivalent at 12V |
|---|---|---|---|---|---|---|
| 0 | Battery Monitor | 12V | 0.14mW | 0.13mW | 0.27mW | 22.5µA |
| 1 | Current Monitor | 2.5V (0A) | 4.1µW | 0.13mW | 0.13mW | 10.8µA |
| 2 | RPM Monitor (optimized) | 1V | 0µW | 0.13mW | 0.13mW | 10.8µA |
| 3 | Temperature (5V design) | 25°C | 1.25mW | 0.13mW | 1.38mW | 115µA |
Total System Power¶
- ADS1115: 0.50mW (150µA @ 3.3V) = 41.7µA equivalent at 12V
- TLV9154IDR: 0.013mW (4µA @ 3.3V) = 1.1µA equivalent at 12V
- Total IC power: 0.51mW = 42.8µA equivalent at 12V
- Total system power: ~2.32mW = 193µA equivalent at 12V (with 5V Channel 3)
Battery Life Analysis (100Ah 12V Battery)¶
- Continuous current draw: ~193µA
- Daily consumption: 4.6mAh (0.005% of capacity)
- Estimated battery life: >50 years (limited by self-discharge, not monitoring system)
- Conclusion: Excellent performance for battery-powered marine applications
Accuracy and Resolution Summary¶
| Channel | Input Range | Engineering Units | ADC Resolution | Best Accuracy | Limitations |
|---|---|---|---|---|---|
| 0 | 0.21V - 65.2V | Battery voltage | 0.0021V | 0.084% | Min 0.21V (op-amp limit) |
| 1 | ±200A | Alternator current | 0.02A | 0.52% | None (within sensor range) |
| 2 | 8Hz - 2640Hz | Engine frequency | Variable | 0.03% | Min 8Hz → 27-479 RPM |
| 3 | 15°C to +125°C | Temperature | 0.1mV | Variable | Cold temp limitation (5V design) |
Design Validation Results¶
Overvoltage Protection Verification¶
✅ Inherent protection confirmed: Single-supply op-amp output cannot exceed 3.3V
✅ No protection diodes required: Eliminates leakage current and temperature drift
✅ Safety margins verified: Op-amp max output (3.3V) < ADS1115 max allowed input
Accuracy Verification¶
✅ No leakage current errors: Op-amp buffer isolates high-impedance voltage dividers
✅ Temperature stability: No Schottky diode temperature coefficients
✅ Precision components: 0.1% resistors eliminate ratio errors
⚠️ Minimum voltage limitations: Op-amp VOL (~10mV) creates measurement dead zones
Noise Performance Verification¶
✅ Low-pass filtering effective: 5nF capacitors remove high-frequency noise before amplification
✅ EMI immunity: Capacitive filtering with proper placement
✅ Ground isolation: Single-point grounding minimizes ground loops
Environmental Suitability¶
✅ Temperature range: -40°C to +85°C (industrial grade components)
✅ Marine environment: Ultra-low power, high accuracy, EMI immune design
✅ Battery compatibility: 129µA total consumption suitable for extended operation
✅ Fault tolerance: Robust protection against high-voltage transients and DC faults
Bill of Materials¶
Shared System Components¶
- U1: TLV9154IDR quad op-amp, SOIC-14 SMD package
- U2: ADS1115IDGSR 16-bit ADC, MSOP-10 SMD package
- C_BYPASS: 100nF ±10%, X7R dielectric, 0603 SMD decoupling capacitor
Channel 0: Battery Voltage Monitor¶
- R1: 1MΩ ±0.1%, 1/8W, 0805 SMD
- R2: 49.9kΩ ±0.1%, 1/8W, 0805 SMD
- C1: 5nF ±10%, X7R, 0603 SMD
Channel 1: Current Monitor¶
- R1: 768kΩ ±0.1%, 1/8W, 0805 SMD
- R2: 768kΩ ±0.1%, 1/8W, 0805 SMD
- C1: 5nF ±10%, X7R, 0603 SMD
Channel 2: RPM Monitor (Optimized Configuration)¶
- R1: Not used (direct connection)
- R2: Not used (direct connection)
- C1: 5nF ±10%, X7R, 0603 SMD
LM2907 Input Circuit: - R_SERIES: 4.7kΩ, 2W (HP122WJ0472T4E), critical for DC fault protection - C_AC1, C_AC2: 10µF/100V, parallel for 20µF total AC coupling - C_FILTER: 6.8nF ±10%, 0603 SMD (5kHz low-pass filter) - R_TERM: 100kΩ ±5%, 1/8W, 0805 SMD - TVS1: SMBJ12CA bidirectional TVS diode
LM2907 Timing Circuit: - C_TIMING1: 10nF ±10%, 0603 SMD (Pin 2) - C_TIMING2: 3 × 1µF ±10%, 0805 SMD parallel (Pin 3, total 3µF) - R_TIMING: 25kΩ ±0.1%, 1/8W, 0805 SMD (Pin 3) - C_BYPASS: 1µF ±10%, 0805 SMD (power supply bypass)
Channel 3: Temperature Monitor (5V Supply Design)¶
- R1: 10kΩ ±0.1%, 1/8W, 0805 SMD (pullup to 5V)
- R2: 10kΩ NTC thermistor (user-supplied)
- Supply: 5V
- C1: 5nF ±10%, X7R, 0603 SMD
Component Selection Notes¶
- Temperature rating: Industrial grade (-40°C to +85°C minimum)
- Precision: ±0.1% resistor tolerance critical for measurement accuracy
- Package size: 0805/0603 SMD for compact layout and thermal stability
- Capacitor dielectric: X7R for temperature stability in filtering applications
- High-power resistor: HP122WJ0472T4E (2W) essential for DC fault protection
- Op-amp selection: TLV9154IDR provides rail-to-rail output for superior low-voltage performance
- Channel 3 limitation: 5V supply creates cold temperature measurement limitations due to ADC range
Customer Application Guidelines¶
Channel 0: Battery Voltage Monitoring¶
- Recommended range: 12V, 24V, or 48V systems
- Resolution: 2.1mV (excellent for battery monitoring)
- Update rate: Suitable for continuous monitoring
- Power impact: 22.5µA at 12V input
- Limitation: Cannot measure below 0.21V (not practically relevant)
Channel 1: Current Monitoring¶
- Compatible sensors: QNHC1K-21 200A hall sensor or equivalent
- Sensor requirements: 2.5V center, ±2V swing for full scale
- Resolution: 0.02A theoretical, ~1A practical accuracy
- Power impact: 10.8µA (minimal)
Channel 2: RPM Monitoring¶
- Input requirements: Minimum 50mV (±50mV) for reliable detection
- Recommended input: 100mV+ for best performance
- Supported configurations:
- Multi-pulse alternator stator (typical marine/automotive)
- Single pulse per revolution (slow engines) -
- Frequency range: 8Hz to 2640Hz
- Engine RPM range:
- Conservative (6-pulse, 1.5:1): 53 RPM to 17,600 RPM
- Aggressive (7-pulse, 2.5:1): 27 RPM to 9,041 RPM
- 1-pulse direct: 479 RPM to 158,400 RPM
- Signal types: Both sine and square waves work identically
- DC fault protection: Safe up to 56V sustained DC input
- Power impact: Eliminated (no voltage divider power consumption)
Channel 3: Temperature Monitor¶
- Current configuration: 10kΩ pullup at 5V supply
- Temperature limitation: 5V supply creates unmeasurable voltages above 3.3V ADC limit for cold temperatures
- Temperature range:
- Current (5V/10kΩ): 15°C to +125°C (cold temperature limitation)
- Potential improvement (3.3V/15kΩ): -40°C to +125°C (full range)
- Resolution: 0.1mV ADC resolution with 0.8-5.8°C temperature resolution depending on curve position
- Power impact: 115µA at 25°C, up to 209µA at 125°C
- Supply requirement: Currently 5V (creates ADC range limitations)
- Design limitation: Cold temperatures below ~15°C cannot be measured due to voltage exceeding 3.3V ADC maximum
System Integration Notes¶
- Total power consumption: 193µA equivalent at 12V (with 5V Channel 3)
- Battery life impact: Minimal (<0.01% daily consumption on 100Ah battery)
- Environmental rating: Suitable for marine applications (-40°C to +85°C)
Recommended Design Improvements¶
Channel 3 Limitation (current 5V design): - Current 5V/10kΩ configuration cannot measure temperatures below ~15°C - Consider 3.3V/15kΩ configuration for full -40°C to +125°C range - Would reduce power consumption by ~40% - Would enable full cold temperature measurement capability