Input capacitance also affects a pole in the feedback path that can cause instability in high-gain, high-frequency applications. By minimizing this input capacitance, you may be able to increase the corresponding pole frequency until it has a negligible effect on the circuit.
Measuring the input capacitance of an op amp isn’t trivial, however; especially if the value is only a few picofarads. Such low values also present difficulties in screening the op amps during production testing. Hence, semiconductor companies often provide only typical values for this parameter, using simulation results and bench measurements on a few known good units. The following discussion can provide a sanity check in the lab by assisting the system-level designer or QA engineer to accurately determine the input capacitance for any op amp.
The direct approach of observing input capacitance on a multimeter isn’t practical below a few nanofarads. A simple yet effective alternative is to insert a large resistor in series with the op-amp input (Figure 1).
Figure 1: A resistor in series with an op amp input enables measurement of the op amp’s input capacitance.
Plotting the frequency response of the resulting first-order lowpass RC filter on a network analyzer (i.e., a Bode plot) lets you calculate the op amp’s input capacitance. Sounds simple, but you must follow precautions to ensure that the measurement accuracy isn’t compromised by stray capacitance in the PC board (PCB) and the test setup.
Follow these tips to minimize stray parasitics:
* Increase the measurement resolution by using only low-capacitance FET probes (<1pF), such as the Tektronix P6245.
* If the series resistor is a surface-mount component, ensure that the board capacitance to ground is as low as possible. (This implies no ground-plane layer beneath the input signal traces and the series resistor.)
* If the series resistor is a through-hole component, bend the input pin so it does not contact the PCB board, and use a short lead length to solder the resistor directly to the op amp input pin.
* Do not use a breadboard in the test setup, because capacitance between the breadboard tracks and jumper wires can degrade the measurement accuracy.
* Use short traces at the input to minimize series inductance.
The hardware recommended for this test setup (Figure 2) includes an Agilent 4395A network analyzer, a Mini-Circuits ZFRSC-2050 power splitter, and a Tektronix P6245 active FET probe.
Figure 2: Test setup for measuring op-amp input capacitance.
First, calibrate the setup with no op amp installed on the PCB. From the resulting Bode plot, you can calculate stray capacitance as Equation 1:
where f1(-3 db) is the corner frequency as measured on the network analyzer with no op amp installed, and RTH1 is the Thevenin-equivalent series resistance. RTH1 is a function of the inserted series resistor, the input termination resistance (50Ω), and the source impedance at the power splitter (50Ω), Equation 2:
Next, install the op amp on the PCB. Since the board’s stray capacitance is in parallel with the op amp’s input capacitance, Equation 1 becomes Equation 3:
where f2(-3 db) is the corner frequency as measured on the spectrum analyzer with the op amp installed, and RTH2 is the Thevenin-equivalent series resistance.
This Thevenin equivalent resistance is a function of the inserted series resistor, the input termination resistance (50Ω), output impedance of the power splitter (50Ω), and the common mode input impedance of the op amp, Equation 4:
The input common mode impedance of an op amp is not accurately known. For a CMOS-input op amp, however, it is fairly easy to select RSERIES << RCM. Then RTH2 ≈ RTH1, and Equation 3 can be rewritten as Equation 5
You can now calculate the op amp’s input capacitance from Equations 1 and 5, and verify the value by repeating the experiment with two different values of series resistor.
To illustrate the method, consider an input-capacitance measurement for the MAX4238 op amp.
Figure 3 shows the amplitude response from Figure 2 using a 200 kΩ series resistor and no op amp installed on the PCB, and Figure 4 shows the amplitude response with the MAX4238 installed.
Figure 3: Amplitude response from Figure 2, with RSERIES = 200 kΩ and no op amp installed on the PCB. The f1(-3dB) frequency is indicated by the downward-pointing arrow.
Figure 4: Amplitude Response from Figure 2, with RSERIES = 200 kΩ and the MAX4238 op amp installed. The f2(-3dB) frequency is indicated by the downward-pointing arrow.
Table 1 summarizes the results, using the frequency-response waveforms and calculations from Equations 1 and 5. As a sanity check, the measurement was repeated with a different series resistor value to demonstrate that a similar result (»4pF) is obtained.
Table 1: Summary of MAX4238 input-capacitance measurements
