Input resistance is the effective opposition to current flow that a circuit, measuring device, or component presents to the signal source connected to its input terminals. In a real circuit, it changes the actual voltage or current delivered by the source by acting as an unintended parallel or series load, effectively altering the very value you are trying to measure or process. People commonly confuse it with input impedance (which includes AC reactance) or output resistance (the source's internal limitation), but input resistance is strictly the DC or real-part load seen looking into the device.
Think of input resistance as a calibrated leak in a pressurized water pipe: the lower the resistance, the more water (current) bleeds off through the meter instead of continuing down the main line, dropping the pressure (voltage) downstream. If you are measuring voltage, you want an infinitely high input resistance so zero current is drawn. If you are measuring current, you want near-zero input resistance so no voltage is dropped.
The Loading Effect: A Worked Numeric Example
To see why this matters on the bench, let us look at the classic voltage divider loading error. Suppose you are monitoring a 12V LiFePO4 battery using a high-impedance voltage divider to minimize quiescent current draw. You use two 1 MΩ resistors (R1 and R2).
In a perfect, unloaded world, the voltage at the midpoint (V_out) is exactly half the source:
- Unloaded V_out: 12V × (1 MΩ / (1 MΩ + 1 MΩ)) = 6.00V
Now, you connect a standard bench digital multimeter (DMM) to verify the midpoint. A typical quality DMM, like the Fluke 87V, has an input resistance of 10 MΩ on its DC voltage ranges. The meter is now in parallel with R2.
- Parallel equivalent of R2 and Meter: (1 MΩ × 10 MΩ) / (1 MΩ + 10 MΩ) = 909.09 kΩ
- Loaded V_out: 12V × (909.09 kΩ / (1 MΩ + 909.09 kΩ)) = 5.71V
Where You Meet Input Resistance in Practice
Different tools and components have vastly different input resistance specifications. Choosing the wrong tool for a high-impedance source guarantees bad data. Here is what you will encounter in the wild:
| Device / Component | Typical Input Resistance | Best Used For |
|---|---|---|
| Standard Bench DMM (e.g., Fluke 87V) | 10 MΩ | General purpose DC/AC voltage checks, low-to-medium impedance circuits. |
| Electrometer / High-Z DMM | 10 GΩ to 100 TΩ | Measuring piezo sensors, ionization chambers, high-Z voltage dividers. |
| Oscilloscope (Standard 1x/10x BNC) | 1 MΩ (parallel with ~15 pF) | Dynamic AC signals, timing analysis. (Note: 1 MΩ loads DC dividers heavily). |
| BJT Op-Amp (e.g., LM358) | ~1 MΩ to 10 MΩ | General buffering, low-cost analog front-ends. Requires low-Z sources. |
| JFET/CMOS Op-Amp (e.g., TL072, LMC6062) | 10^12 Ω (1 TΩ) or higher | Audio inputs, piezo buffering, high-impedance sensor amplification. |
| Microcontroller ADC (Direct Pin) | Highly variable (often < 1 MΩ effective) | Low-impedance sources only (< 10 kΩ recommended for accurate SAR sampling). |
Real-World Scenario: The Battery Monitor That Read 1.0V
Abstract math is one thing; bricking a weekend project is another. Here is a real-world scenario walkthrough that highlights what happens when input resistance is ignored in embedded systems.
The Setup
A hobbyist is building a 48V solar battery monitor using an ESP32 development board. To step the 48V down to the ESP32's 3.3V ADC limit, they build a voltage divider using R1 = 4.7 MΩ and R2 = 330 kΩ. They chose megaohm resistors to keep the continuous current draw under 10 µA, preserving battery life.
The Numbers
- Expected ADC Voltage: 48V × (330 kΩ / (4.7 MΩ + 330 kΩ)) = 3.14V
- ESP32 ADC Input Resistance: The ESP32's internal SAR ADC does not have a simple, infinite DC input resistance. Due to internal multiplexer leakage and the sampling capacitor charge time, the effective input resistance can drop significantly, often behaving like a 1 MΩ to 2 MΩ load depending on the attenuation setting.
The Outcome
The ESP32 firmware reads the ADC pin, calculates the battery voltage, and reports it to the dashboard as 14.2V. The hobbyist assumes the battery is dead or the ESP32 is fried.
What Went Wrong
The ESP32's effective 1.5 MΩ input resistance was placed in parallel with the 330 kΩ R2 resistor. The equivalent resistance of R2 dropped from 330 kΩ down to roughly 270 kΩ. Worse, the high 4.7 MΩ source impedance meant the ADC's internal sampling capacitor could not fully charge during the brief sampling window, compounding the DC resistive error with a dynamic charge-transfer error. The ADC saw roughly 0.9V to 1.1V instead of 3.14V.
Input Resistance vs. Input Impedance
If you read datasheets, you will frequently see input impedance instead of resistance. Here is the practical difference:
- Input Resistance (R_in): The purely real, DC opposition to current. It dictates how much steady-state DC current the device draws from the source. It is measured in Ohms (Ω).
- Input Impedance (Z_in): The total opposition to alternating current (AC), combining resistance (R) and reactance (X) from internal capacitance or inductance. It is frequency-dependent.
For example, a standard oscilloscope has an input resistance of 1 MΩ, but its input impedance at 100 MHz might be only a few hundred ohms due to the 15 pF parallel input capacitance. If you are measuring DC or 60 Hz mains, you only care about input resistance. If you are measuring RF or high-speed digital edges, input impedance (and capacitance) is the metric that will ruin your signal integrity.
FAQ: Common Bench Questions
Why does my cheap multimeter read differently than my Fluke on the same circuit?
While most modern DMMs, even budget ones, use a 10 MΩ input divider network, cheap meters often suffer from high input bias currents, poor PCB leakage paths, and uncalibrated parallel resistance. If you are measuring a source with an output impedance above 100 kΩ, a budget meter's actual input resistance might effectively be 8 MΩ or 9 MΩ due to internal loading, skewing the reading compared to a precision 10 MΩ Fluke or Keysight meter.
Can I just use a higher value resistor in my voltage divider to fix loading?
No, that makes it worse. Higher divider resistances increase the source's output impedance, making the circuit more susceptible to the loading effect of whatever is measuring it. To fix loading, you must either lower the divider resistances (increasing power draw) or insert a unity-gain buffer op-amp with a high input resistance and low output resistance between the divider and the measuring device.
Does input resistance matter when measuring current?
Yes, but the goal flips. When measuring current, the meter is placed in series with the load. You want the meter's input resistance (often called 'burden voltage' or shunt resistance) to be as close to zero ohms as possible. If your DMM's current shunt has 1 Ω of resistance and you are measuring a 3.3V logic circuit drawing 1A, the meter drops 1V, leaving only 2.3V for your circuit, potentially causing a brownout.
