OPA830 datasheet(22/56 Pages) TI

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Operating the OPA838 in inverting mode with higher RF values increases response peaking due to the loss of

phase margin effect. In the inverting case, a pair of capacitors can flatten the response at the cost of lower

closed-loop bandwidth. Figure 7-4 shows an example with a 20-kΩ RF value at an inverting gain of –5 V/V

(noise gain = 6 V/V) with optional capacitors (CF and C G ). Figure 7-4 shows optional bias current cancellation

elements on the noninverting input. The total resistance value matches the parallel combination of RG || R F ,

which reduces the DC output error term due to bias current to IOS × RF. The 10-nF capacitor is added across

the larger part of this bias current canceling resistance to filter noise and the 20 Ω is split out to isolate the

capacitor self resonance from the noninverting input. Figure 7-5 illustrates the small-signal response shape with

and without these capacitors. The feedback capacitor (CF), is selected to set a desired closed-loop bandwidth

with RF. CG is added to ground to shape the noise gain up over frequency to be greater than or equal to 6 V/V

at higher frequencies. In this example, that higher frequency noise gain is 1 + 6 / 1.2 = 6 V/V, adding the 1-pF

device common-mode capacitance to the external 5 pF. Using the capacitors to set the feedback ratio removes

the pole produced in the feedback driving from purely resistive source to the inverting parasitic capacitance.

RF 20 NŸ

RG 4.02 NŸ

RG 4.02 Ÿ

VOUT

VEE

VCC

VEE

VCC

V1 2.5 V

V2 ±2.5 V

Bias current

cancellation with

resistor noise

filtering

Inverting with

High Rf values

CG 5 pF

RM 3.24 NŸ

C1 10n

Input

CF 1.2 pF

Figure 7-4. G = –5 V/V With Optional Compensation

Frequency (Hz)

-15

-12

-9

-6

-3

10k

100k

10M

100M

D070

No caps

With caps

Figure 7-5. Inverting Response With and Without Compensation

OPA838

SBOS867D – AUGUST 2017 – REVISED SEPTEMBER 2024

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