Follow Us QUESTION:
While measuring op amp input capacitance, what should I focus on?
Answer:
You must ensure that the measurement accuracy is not degraded by the stray capacitance and inductance of the PCB or test setup. You can minimize these issues by using probes with low capacitance, using short lines on the PCB, and avoiding huge ground planes below the signal traces.
Operational amplifiers are used in a wide variety of electronic circuits. Their task is to amplify small electrical voltages for further signal processing. Applications such as smoke detectors, photodiode transimpedance amplifiers, medical instruments, and even industrial control systems require the lowest possible op amp input capacitance because, among other things, this affects the noise component, which in turn affects system stability, especially for systems with high frequencies and gains.
To maximize the accuracy of a corresponding circuit, it is necessary to know the input capacitance of the op amp. Data sheets, however, often do not provide this information, so it must be independently determined. This can be difficult because the input capacitance in many cases is only a few pF.
Table 1 lists a few different examples of operational amplifiers and their respective input capacitance values.
| Op Amp | Op Amp Type | Input Capacitance |
| LT1792 | JFET input op amp | 14 pF |
| LT1813 | Low noise op amp | 2 pF |
| AD826 | High speed dual op amp | 1.5 pF |
| ADA4097-1 | Low input bias current/precision op amp | 3 pF |
| AD8009 | Current feedback op amp | 2.6 pF |
How to Determine Input Capacitance
An easy way to determine the input capacitance of an op amp is to add a resistor in series (RSERIES) with the op amp input as shown in Figure 1. This yields a first-order low-pass filter with a frequency response that can be recorded by a network analyzer. The input capacitance can be calculated from the frequency response. The resistance RSERIES is typically in the range of 10 kΩ to 100 kΩ.
When recording the frequency response, you must ensure that the measurement accuracy is not degraded by the stray capacitance and inductance of the PCB or test setup.
A high measurement resolution should be selected for minimal stray capacitance. The use of FET probes of low capacitance (<1 pF) is advisable.
The PCB capacitance with respect to ground should also be kept as low as possible. This can be achieved by ensuring that there is no ground plane below the signal traces and the series resistor.
In addition, the shortest possible lines and (resistor) leads should be used to avoid additional sources of error such as series and parasitic inductance.
Figure 2 shows a possible test setup, using a network analyzer and a power splitter.
The power splitter has the task of dividing the signal. The signal 1:1 is fed unchanged to the input of the network analyzer and is also passed through the inserted low-pass filter to the op amp input. The network analyzer then generates the frequency response from the difference between these two signals.
For the measurement itself, the stray capacitance CSTRAY needs to be determined. For this, the signal is applied without the op amp on the board. From the resulting Bode plot, CSTRAY is calculated as shown in Equation 1:
f1(–3 dB) is the –3 dB corner frequency measured with the network analyzer without an operational amplifier, and RTH1 is a function of the inserted series resistance (RSERIES), the input termination resistance (50 Ω), and the 50 Ω source impedance at the power splitter (Thévenin equivalent):
Next, the op amp is placed on the PCB.
Because the stray capacitance of the PCB is in parallel with the input capacitance of the op amp, Equation 1 is supplemented with CIN as shown in Equation 3:
This time, f2(–3 dB) is the –3 dB corner frequency measured by the network analyzer with an op amp and RTH2 is a function of the inserted series resistance, the input termination resistance (50 Ω), the output impedance of the power splitter (50 Ω), and the common-mode input impedance of the op amp (RCM):
In general, for op amps with CMOS inputs, RSERIES << RCM. Therefore, RTH2 ≈ RTH1 and Equation 3 can be rewritten as shown in Equation 5:
The input capacitance of the operational amplifier can then be determined using equations 1 and 5.
Conclusion
The input capacitance of an operational amplifier can be difficult to measure. It often lies in the pF range and parasitic effects in the test setup distort the result. With a small test setup and the appropriate measuring equipment consisting of a network analyzer and a power splitter, it is easy to determine the input capacitance by first determining the stray capacitance (error capacitors in the test setup) and then determining the combined capacitance (error capacitors and input capacitance) of the op amp circuit via the frequency response. With the equations shown earlier, the actual input capacitance of the operational amplifier can be calculated.
