OPA830 datasheet(31/56 Pages) TI

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8.1.4 Output Noise Calculations

The decompensated voltage feedback of the OPA838 op amp offers among the lowest input voltage and current

noise terms for any device with a supply current less than 1 mA. Figure 8-3 shows the op amp noise analysis

model that includes all noise terms. In this model, all the noise terms are shown as noise voltage or current

density terms in nV/√Hz or pA/√Hz.

OPA838

ENI

IBN

IBI

4kTR

4kT = 1.6E ± 20J

at 290° K

4kT

ERS

Figure 8-3. Op-Amp Noise-Analysis Model

The total output spot noise voltage is computed as the square root of the squared contributing terms to the

output noise voltage. This computation is adding all the contributing noise powers at the output by superposition,

then taking the square root to return to a spot noise voltage. Equation 3 shows the general form for this output

noise voltage using the terms presented in Figure 8-3.

4kTR

I R

4kTR NG

(3)

Dividing this expression by the noise gain (NG = 1 + RF / RG) gives the equivalent input-referred spot noise

voltage at the noninverting input, as shown in Equation 4.

I R

4kTR

(4)

Using the resistor values shown in Table 8-1 with RS = 0 Ω results in a constant input-referred voltage noise of

2.86 nV/√Hz. Reducing the resistor values brings this number closer to the intrinsic 1.9 nV/√Hz of the OPA838.

Adding RS for bias current cancellation in noninverting mode adds the noise from RS to the total output noise;

see Equation 3. In inverting mode, bypass the RS bias-current cancellation resistor with a capacitor for the best

noise performance.

8.2 Typical Applications

8.2.1 High-Gain Differential I/O Designs

A high-gain differential-to-differential I/O circuit can be used to drive a second-stage FDA or a differential-to-

single-ended stage. This circuit is frequently used in applications where high input impedance is required (for

example, if the source cannot be loaded). Figure 8-3 illustrates an example design where the differential gain

is 41 V/V. An added element between the two RG resistors increases the noise gain for the common-mode

feedback. Make sure to provision for the added element; otherwise, a decompensated VFA (such as the

OPA838) often oscillates. With only the RG elements in the differential I/O design, the common-mode feedback

is unity-gain and often causes high-frequency, common-mode oscillations. To resolve this issue, split the RG

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OPA838

SBOS867D – AUGUST 2017 – REVISED SEPTEMBER 2024

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