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Yes, the ADS1115 can return negative readings when it measures a negative difference between two inputs. No, that does not mean you can connect a voltage below the ADC’s ground pin directly to an analog input. In differential mode it reports AINP − AINN, so both pins can be above ground while the result is negative. A truly below-ground signal must be level-shifted or handled by a circuit designed for bipolar inputs.
First identify what “negative” means in your circuit
Three situations are often confused:
- Negative differential voltage: AINP is lower than AINN, so AINP − AINN is negative. This is supported when each pin remains within its permitted voltage range.
- Negative voltage relative to ground: an input pin is below the ADS1115 GND pin. A normal single-supply ADS1115 circuit cannot accept this directly.
- A sensor described as bipolar: the sensor may produce positive and negative values, but its output might already be biased above ground. Check the actual voltage at each ADC pin, not just the sensor’s stated signal range.
For example, AINP = 2.0 V and AINN = 3.0 V produces a −1.0 V differential result while both physical inputs are positive. By contrast, connecting −2.5 V to AIN0 relative to GND is an out-of-range input on a normal single-supply setup. TI’s ADS1115 datasheet describes the differential operation and the input-voltage constraints.
How differential readings become negative
The ADC subtracts the negative input from the positive input:
VIN = VAINP − VAINN
| AINP | AINN | Measured difference |
|---|---|---|
| 3.0 V | 2.0 V | +1.0 V |
| 2.0 V | 3.0 V | −1.0 V |
| 2.5 V | 2.5 V | 0 V |
| 0.2 V | 0.8 V | −0.6 V |
The sign is determined by the order of subtraction. The ADS1115 provides four differential pair selections: AIN0 − AIN1, AIN0 − AIN3, AIN1 − AIN3, and AIN2 − AIN3. The other input modes measure AIN0, AIN1, AIN2, or AIN3 relative to GND.
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In single-ended mode, an ordinary input is measured from 0 V toward the positive supply, subject to the selected range and supply-related limits. It is not the mode for measuring a signal that crosses below ground or for reporting the sign of the difference between two pins. A small negative code near zero can occur because of device offset; that is not a usable negative single-ended range.
Wire the inputs and choose the correct library call
For a differential measurement, connect the two signal nodes to one of the supported pairs and make sure both pin voltages are legal relative to the ADS1115’s ground and supply. A negative result means the pin named first in the pair is lower than the second. If using AIN3 as a shared comparison point, remember that this does not remove the individual input-voltage limits or provide the same common-mode noise rejection as a conventional differential measurement.
With the Adafruit ADS1X15 library, the differential functions include readADC_Differential_0_1(), readADC_Differential_0_3(), readADC_Differential_1_3(), and readADC_Differential_2_3(). The library documents these as signed readings. Its ADS1115 API reference describes the calls and computeVolts() method.
A single-ended call such as readADC_SingleEnded(0) reads AIN0 relative to ground; it does not substitute for a differential reading.
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Arduino example: read a signed differential value
#include <Wire.h>
#include <Adafruit_ADS1X15.h>
Adafruit_ADS1115 ads;
void setup() {
Serial.begin(115200);
if (!ads.begin()) {
Serial.println("ADS1115 not found");
while (1) {
delay(10);
}
}
ads.setGain(GAIN_ONE); // ±4.096 V nominal PGA range
}
void loop() {
int16_t counts = ads.readADC_Differential_0_1();
float volts = ads.computeVolts(counts);
Serial.print("Signed counts: ");
Serial.print(counts);
Serial.print(" Differential voltage: ");
Serial.print(volts, 6);
Serial.println(" V");
delay(250);
}The example uses AIN0 − AIN1. If the displayed sign is opposite to the intended polarity, verify which signal is connected to each pin before changing the code. Swapping the pair reverses the sign; negating the result in software is appropriate only when it matches the intended definition of positive.
Interpret raw conversion data as signed two’s complement
The conversion register is a 16-bit two’s-complement value. If a library returns a negative differential result as a large positive number, the raw word may have been kept unsigned or not sign-extended correctly.
| Raw 16-bit word | Signed value |
|---|---|
0x0000 |
0 |
0x0001 |
+1 |
0xFFFF |
−1 |
0xFFFE |
−2 |
0x7FFF |
+32767 |
0x8000 |
−32768 |
For manually read I²C bytes in Arduino C++, combine the bytes and cast the result to a signed 16-bit type:
uint16_t rawWord = (uint16_t(highByte) << 8) | lowByte;
int16_t signedCounts = (int16_t)rawWord;Use int16_t for the signed count rather than uint16_t. The ADS1115 uses two’s complement, not one’s complement.
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Convert counts to volts and select the PGA range
For the ADS1115, the nominal least-significant-bit size is the selected full-scale range (FSR) divided by 216. Multiply the signed count by that LSB size to get the measured differential voltage. The Adafruit library’s computeVolts(counts) uses the selected gain setting.
| Nominal PGA range | Nominal LSB size |
|---|---|
| ±6.144 V | 187.5 µV |
| ±4.096 V | 125 µV |
| ±2.048 V | 62.5 µV |
| ±1.024 V | 31.25 µV |
| ±0.512 V | 15.625 µV |
| ±0.256 V | 7.8125 µV |
For example, at the ±4.096 V setting, a count of −800 corresponds nominally to −800 × 125 µV = −0.100 V. At the ±2.048 V setting, −16,000 counts corresponds nominally to −1.000 V. Use the selected PGA range for this calculation, not VDD alone; TI specifies the FSR values in its datasheet.
Choose the narrowest range that safely contains the largest expected differential signal, allowing room for offset, tolerances, transients, and overshoot. Illustrative choices are ±0.256 V for a signal up to about ±100 mV, ±0.512 V for about ±400 mV, ±2.048 V for about ±1.5 V, and ±4.096 V for about ±3 V. These are range-selection examples, not guarantees about the input pins.
The ±6.144 V setting is a PGA scaling range, not permission to drive an input pin to 6.144 V. For a 3.3 V-powered part, selecting ±4.096 V does not make 4.096 V a safe input voltage; the supply-related pin limits still apply. The ADS1115 itself operates from 2.0 V to 5.5 V, and TI lists a maximum data rate of 860 samples per second on its product page.
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Check differential range and each pin’s voltage separately
A valid differential reading requires more than a difference inside the chosen FSR. Check all of the following:
- The AINP − AINN difference fits inside the selected PGA range.
- Each analog input stays within the device’s permitted voltage range relative to GND and VDD.
- The circuit’s common-mode voltage is suitable for the ADC and for any connected amplifier or sensor.
- Neither input is driven below ground or excessively above the positive rail.
This matters for current-shunt measurements: the voltage across the shunt may be only a few millivolts, but both shunt terminals can sit at a much higher common-mode voltage. A small differential voltage does not make an excessive pin voltage safe. TI warns that extended exposure to input voltages roughly 300 mV beyond the supply rails can damage the device and discusses current limiting for overvoltage protection in the datasheet.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Measure a signal that genuinely goes below ground
For a sensor output of −2.5 V to +2.5 V relative to system ground, do not connect the bipolar signal directly to a single-supply ADS1115 input. First translate it into the ADC’s legal input range.
Bias or level-shift the signal
One possible transformation in a 5 V system is to shift −2.5 V to +2.5 V into 0 V to 5 V. The software then removes the offset: originalVoltage = measuredBiasedVoltage − 2.5 V. A midpoint can come from a divider, reference, or level-shifting amplifier. A divider may be adequate in some circuits, but buffer the bias when source impedance, loading, or accuracy makes its voltage unstable. Adafruit’s signal-connection guidance likewise says inputs must stay between ground and VCC and describes offsetting negative sources.
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Use a differential or instrumentation amplifier
An amplifier stage can shift and scale the bipolar signal while keeping the ADC pins in range. This is often preferable when the source impedance is high, the signal has a substantial common-mode component, or noise rejection and accuracy matter. Check the amplifier’s supply, input common-mode range, output swing, offset, and noise against the full circuit; not every amplifier can produce the desired range on the available supply.
Choose a bipolar-input converter or suitable supply architecture
If the application requires direct negative input voltages, select an ADC and analog front end designed for that requirement, or design an appropriate negative-supply circuit. A signed differential result alone does not make the ADS1115 a bipolar-input ADC. TI’s support response on negative-voltage inputs makes this distinction; the datasheet remains the primary source for device limits.
Troubleshoot unexpected readings
| Symptom | What to check | What to do |
|---|---|---|
| Reading is always positive | Are you using a single-ended function, measuring only relative to ground, or using the wrong AINP/AINN order? | Use the intended differential pair and confirm its subtraction order. |
| Negative value appears as a huge positive integer | Is the conversion word stored as unsigned or missing sign extension? | Use int16_t or explicitly interpret the 16-bit word as two’s complement. |
| Reading becomes zero for a negative input | Was a below-ground signal connected directly in single-ended mode? | Do not treat zero as a valid measurement. Disconnect the out-of-range signal and add suitable conditioning or use a bipolar-capable input circuit. |
| Sign is backwards | The selected function reports AINP − AINN; are the input wires in the expected order? | Swap the pair or apply a documented software sign reversal after confirming polarity. |
| Reading clips near a limit | Check the PGA range, actual differential voltage, transients, supply-limited pin range, and common-mode voltage. | Choose an appropriate range and bring every input within its legal voltage limits. TI specifies clipping at 7FFFh for positive overrange and 8000h for negative overrange. |
| Noisy readings near zero | Consider signal size relative to ADC offset and noise, PGA range, source impedance, wiring, common-mode stability, and data rate. | Try a narrower FSR, shorter or differential wiring, appropriate input filtering, a buffer for high source impedance, or a lower data rate where latency allows. Averaging should not conceal clipping or instability. |
Do not test an out-of-range negative source by connecting it directly and relying on the ADC to clamp it. For protection against unexpected transients, design current limiting and other input protection around the datasheet limits and the circuit’s required accuracy.
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