A pull-down resistor is useful when an op-amp input could otherwise float, but it is not automatically required. Its job may be to provide a bias-current return path, establish a switch’s default low state, or bias an AC signal to a reference voltage. The correct connection and value depend on the circuit type.
First identify what circuit you have
Before choosing a resistor, determine the op-amp part number, supply voltage, input pin, resistor location, resistor value, presence of a coupling capacitor, and whether negative feedback is connected.
| Situation | What the resistor does | Typical destination |
|---|---|---|
| AC-coupled or high-impedance input | Provides a DC return path for bias and leakage currents | Ground or a mid-supply reference |
| Switch or open sensor | Creates a defined inactive logic-low state | Ground |
| Inverting amplifier | Optional bias-current compensation, not usually a signal pull-down | Ground or the circuit reference at the non-inverting input |
| Threshold detector | May be an inappropriate attempt to replace a comparator | Depends on the comparator design |
What a pull-down resistor actually does
A pull-down connects a node to ground or another low reference when no stronger source drives it. A typical arrangement is:
signal or switch
|
+------ op-amp input
|
RPD
|
GND
When the source is disconnected or high impedance, the input moves toward the reference. When the source drives the node, it must supply the pull-down current. Analog Devices describes this resistor as an input-bias-current return path: AN-937.
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Why a high-impedance input still needs a DC path
“High input impedance” does not mean zero current. Real op-amps have input bias current, offset current, leakage, protection structures, and input capacitance. A coupling capacitor blocks DC, so the resistor on the op-amp side of that capacitor gives those currents somewhere to flow.
A first-order error estimate is VERROR ≈ IB × RSOURCE. For 10 nA through 100 kΩ, the error is about 1 mV. For 1 µA through 1 MΩ, it is 1 V. Use the maximum bias-current specification over the actual temperature, supply, and common-mode range; do not rely on a typical value.
Analog Devices discusses this mechanism and common 100 kΩ to 1 MΩ return-resistor ranges in AN-937 and precision offset effects in Minimize Voltage Offsets in Precision Amplifiers.
Where the resistor belongs
AC-coupled non-inverting amplifier
VIN -- C --+-------- (+)
|
RPD
|
GND or VREF
VOUT -- RF --+
|
(−)
|
RIN
|
GND or VREF
Place the resistor after the coupling capacitor, directly at the op-amp input node. If the circuit runs from one supply and the signal is bipolar, connect it to VREF, commonly near half the supply, rather than automatically connecting it to ground.
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Voltage follower
VIN --------+-------- (+)
|
RPD
|
GND
VOUT -------------- (−)
This resistor is useful only when the source can be disconnected or become high impedance. A low-impedance source already establishes the input voltage, so an added resistor may merely load it.
Inverting amplifier
VIN -- RIN --+-- (−)
|
RF
|
VOUT
(+)
|
RB
|
GND or VREF
The inverting input already has a DC path through RIN. RB at the non-inverting input is an optional impedance-balancing resistor intended to reduce bias-current offset. A traditional starting point is RB = RIN || RF. With 10 kΩ and 100 kΩ, that is approximately 9.09 kΩ, so 9.1 kΩ is a practical starting value.
Do not add this resistor automatically. Analog Devices notes that modern CMOS, JFET, and bias-current-cancelled amplifiers may gain little and can suffer additional noise, offset, or stability problems: StudentZone, June 2017 and Common Problems When Designing Amplifier Circuits.
Switch or sensor input
+5 V or signal
|
switch
|
+-------- input
|
RPD
|
GND
When the switch is open, the resistor holds the input low. When it closes, the source must sink IPULLDOWN = VSIGNAL / RPD. A 5 V signal through 10 kΩ draws 0.5 mA; through 100 kΩ it draws 50 µA.
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Ground or a mid-supply reference?
Ground is normally appropriate for a dual-supply circuit or a unipolar input that should default to 0 V. In a 0 V/+5 V single-supply AC amplifier, grounding the input after a coupling capacitor prevents a negative-going waveform from being represented correctly. Bias the node to a quiet reference near 2.5 V instead:
VIN -- capacitor -- input node
|
R
|
VREF ≈ VS/2
A resistor divider can create the reference, but its impedance and supply noise matter. Bypass or buffer it when the signal path requires a stable reference. Analog Devices covers single-supply biasing and example divider values in AN-581.
Choosing the resistor value
1. Set a bias-current error limit
Use RPD ≤ VERROR,ALLOW / IB(MAX). If the maximum allowed error is 5 mV and maximum bias current is 50 nA, the resistor should be no greater than 100 kΩ.
2. Check source loading
The resistor draws current whenever the source is high. A 10 kΩ resistor at 5 V draws 0.5 mA; a 1 MΩ resistor draws 5 µA. For a source with output resistance RS, the loaded input is approximately VIN = VS × RPD/(RS + RPD). This loading also changes a sensor or divider’s calibration; see DigiKey’s discussion of bias-current and divider loading at Decoding Op-Amp Datasheets: Input Bias Current.
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3. Check the RC corner
With an input coupling capacitor, fC = 1/(2πRPD C). A 100 kΩ resistor and 1 µF capacitor produce approximately 1.59 Hz. Increasing resistance lowers the cutoff but makes the node more vulnerable to leakage, interference, and slow settling.
4. Allow for noise and leakage
Resistor Johnson-noise density is en = √(4kTR), so higher resistance produces more voltage noise. Op-amp current noise also becomes voltage noise through source resistance. Analog Devices explains these trade-offs at What Should I Know About Op Amp Noise?. At hundreds of kilohms or megohms, PCB contamination, humidity, cable leakage, switch leakage, and even a measurement probe can materially affect the voltage.
Useful starting ranges
| Use case | Starting range | Main constraint |
|---|---|---|
| Switch or low-leakage sensor | 4.7 kΩ–100 kΩ | Switch current, leakage, and noise margin |
| General breadboard input bias | 10 kΩ–100 kΩ | Loading and noise |
| AC-coupled audio or sensor | 100 kΩ–1 MΩ | Bias error, leakage, and noise |
| Very low-power design | 1 MΩ–10 MΩ | Leakage, interference, and slow RC response |
| Precision DC measurement | Usually lower than a casual pull-down | Offset, noise, and source loading |
These are starting points, not universal recommendations.
Worked examples
AC-coupled 5 V sensor
Use a 2.5 V reference, a 1 µF coupling capacitor, and a 100 kΩ return resistor. The input corner is about 1.59 Hz. Confirm that the op-amp’s input common-mode range includes the signal’s entire excursion around 2.5 V.
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Switch with a 100 µA current limit
At 5 V, RPD ≥ 5 V / 100 µA = 50 kΩ. A 100 kΩ resistor draws 50 µA when the switch drives the input high, provided leakage and noise margins remain acceptable.
Inverting amplifier
For RIN = 10 kΩ and RF = 100 kΩ, the impedance-balancing starting value is 9.1 kΩ. Verify whether the selected op-amp benefits from it before fitting the part.
Why the output is stuck at a rail
- The resistor is on the wrong side of a coupling capacitor, disconnected, or connected to the wrong op-amp pin.
- The op-amp is being used open-loop; tiny differential voltages drive it toward a rail.
- The input common-mode range is exceeded, even though the output is described as rail-to-rail.
- The output swing is insufficient for the load and supply voltage.
- A supply pin, ground connection, or bypass capacitor is wrong.
- The feedback resistor is open, miswired, or in the wrong breadboard row.
- The source and pull-down form an unintended divider.
- The device is actually a comparator or an open-drain/open-collector output that needs a pull-up.
- The circuit is oscillating; a multimeter may show only its average value.
Rail-to-rail input and rail-to-rail output are separate specifications. Check both under the actual supply, load, temperature, and common-mode conditions.
A practical troubleshooting sequence
- Mark the
+input,−input, output, supply pins, and both sides of every capacitor. - Disconnect the signal source and measure the input node. It should sit near ground or
VREF, not drift randomly. - Verify the resistor value out of circuit or against its marking; 10 kΩ, 100 kΩ, 1 MΩ, and 100 Ω are commonly confused.
- Apply a known voltage with a potentiometer or divider and check the expected closed-loop equation.
- Calculate source loading with
IPD = VS/RPDand the divider equation above. - Check the gain: non-inverting
AV = 1 + RF/RG; invertingAV = −RF/RIN. - Measure
VREFwhile the circuit is operating. A high-impedance divider may move under load. - Use an oscilloscope to look for oscillation, clipping, switching spikes, slow RC charging, or noise on the wrong DC level.
Pull-down, pull-up, op-amp, or comparator?
A pull-down establishes a default voltage; it does not create hysteresis or turn an op-amp into a logic device. Use negative feedback when you need linear gain, filtering, buffering, or an analog output. Use a comparator when you need a definite high/low state, fast switching, guaranteed hysteresis behavior, or an open-drain/open-collector interface.
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|---|---|
| Analog gain or buffer | Op-amp with negative feedback |
| Threshold and logic output | Comparator |
| Noisy or slow input | Comparator with positive-feedback hysteresis |
| Open-drain/open-collector output | External pull-up to create the high state |
TI explains the behavioral and output-stage differences in Op-Amp vs Comparators. If a comparator-like circuit is unavoidable with an op-amp, expect saturation and recovery behavior to be less controlled.
Final checklist
- Is the input ever disconnected or capacitor-coupled?
- Is there a DC path to ground or the correct
VREF? - Is the resistor after the coupling capacitor?
- Is source loading acceptable?
- Is
IB × Rinside the error budget? - Are resistor noise, leakage, and settling time acceptable?
- Are input common-mode and output-swing limits satisfied?
- Is negative feedback present and intact?
- Should the circuit use a comparator and hysteresis instead?
The Bottom Line
Use a pull-down when the op-amp input needs a defined DC path or default-low state. Choose its value from bias-current error, source loading, RC response, noise, and leakage—not from a universal 10 kΩ or 100 kΩ rule. In a single-supply AC circuit, the correct destination is often a buffered mid-supply reference rather than ground.
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