OPA830 datasheet(29/56 Pages) TI

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Table 8-1 lists the recommended resistor values from target gains of 6 V/V to 20 V/V. This table controls the

RF and RG values to set the resistor noise contribution at approximately 40% of the total output noise power.

This increases the spot noise at the output over what the op amp voltage noise produces by 20%. Lower values

reduce the output noise of any design at the cost of more power in the feedback circuit. Using the TINA model

and simulation tool shows the impact of different resistor value choices on response shape and noise.

Table 8-1. Noninverting Recommended Resistor Values

TARGET

AVERAGE

RF (Ω)

RG (Ω)

ACTUAL GAIN (V/V)

GAIN (dB)

1000

200

15.56

1180

196

7.02

16.93

1370

196

7.99

18.05

1540

191

9.06

19.15

1690

187

10.04

20.03

1870

187

20.83

2050

187

11.96

21.56

2210

182

13.14

22.37

2370

182

14.02

22.94

2550

182

15.01

23.53

2740

182

16.05

24.11

2870

178

17.12

24.67

3090

182

17.98

25.09

3240

178

19.20

25.67

3400

178

20.1

26.06

3570

178

21.06

26.47

8.1.2 Inverting Amplifier

The OPA838 can be used as an inverting amplifier with signal input to the inverting input (VIN–) through the

gain-setting resistor (RG.) Figure 7-2 illustrates a basic block diagram of the circuit.

If VIN = VREF + VSIG, and the noninverting input is dc biased to VREF, the output of the amplifier is calculated

according to Equation 2:

SIG

REF

= V

+ V

OUT

(2)

The signal gain of the circuit

G =

and VREF provides a reference point around which the input and output

signals swing. For bipolar-supply operation, VREF is often GND. The output signal is 180° out-of-phase with the

input signal in the pass band of the application. Figure 8-2 illustrates the 50-Ω input matched configuration used

for the inverting characterization plots. In this case, an added termination resistor is placed in parallel with the

input RG resistor to provide an impedance match to 50-Ω test equipment. Table 8-2 lists the suggested values for

RF, RG, and RT for inverting gains from –6 V/V to –20 V/V.

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OPA838

SBOS867D – AUGUST 2017 – REVISED SEPTEMBER 2024

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