The OPA836 and OPA2836 devices are designed for the nominal value of R_F to be 1 kΩ in gains other than +1. This gives excellent distortion performance, maximum bandwidth, best flatness, and best pulse response, but it also loads the amplifier. For example; in gain of 2 with R_F = R_G = 1 kΩ, R_G to ground, and V_OUT = 4 V, 2 mA of current will flow through the feedback path to ground. In gain of +1, R_G is open and no current will flow to ground. In low-power applications, it is desirable to reduce the current in the feedback by increasing the gain-setting resistors values. Using larger value gain resistors has two primary side effects (other than lower power) due to their interaction with parasitic circuit capacitance:

• Lowers the bandwidth
• Lowers the phase margin
  • This causes peaking in the frequency response
  • This also causes overshoot and ringing in the pulse response

Figure 8-3 shows the small-signal frequency response on OPA836EVM for noninverting gain of 2 with R_F and R_G equal to 1 kΩ, 10 kΩ, and 100 kΩ. The test was done with R_L = 1 kΩ. Due to loading effects of R_L, lower R_L values may reduce the peaking, but higher values will not have a significant effect.

Figure 8-3 Frequency Response With Various Gain-Setting Resistor Values

As expected, larger value gain resistors cause lower bandwidth and peaking in the response (peaking in the frequency response is synonymous with overshoot and ringing in the pulse response). Adding 1-pF capacitors in parallel with R_F helps compensate the phase margin and restores flat frequency response. Figure 8-4 shows the test circuit.

Figure 8-4 G = 2 Test Circuit for Various Gain-Setting Resistor Values