AD8132ARZ-R7 datasheet(22/32 Pages) AD

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AD8132

Rev. F | Page 22 of 32

OTHER β2 = 1 CIRCUITS

The preceding simple configuration with β2 = 1 and its gain of

2 is the highest gain circuit that can be made under this condition.

Since β1 was equal to 0, only higher β1 values are possible. The

circuits with higher values of β1 have gains lower than 2. However,

circuits with β1 equal to 1 are not practical because they have

no effective input and result in a gain of 0.

To increase β1 from 0, it is necessary to add two resistors in a feed-

back network. A generalized circuit that has β1 with a value higher

than 0 is shown in Figure 69. A couple of different convenient

gains that can be created are a gain of 1, when β1 is equal to 1/3,

and a gain of 0.5, when β1 equals 0.6.

With β2 equal to 1 in these circuits, VOCM serves as the refer-

ence voltage that measures the input voltage and the individual

output voltages. In general, when VOCM is varied in circuits

with unmatched feedback networks, a differential output

signal is generated that is proportional to the applied VOCM

voltage.

VARYING β2

Though the β2 = 1 circuit sets β2 to 1, another class of simple

circuits can be made that sets β2 equal to 0. This means that

there is no feedback from +OUT to −IN. This class of circuits

is very similar to a conventional inverting op amp. However,

the AD8132 circuits have an additional output and common-

mode input that can be analyzed separately (see Figure 71).

With −IN connected to ground, +IN becomes a virtual ground

in the sense that the term is used for conventional op amps. Both

inputs must maintain the same voltage for equilibrium operation;

therefore, if one is set to ground, the other is driven to ground.

The input impedance can also be seen to be equal to RG, just as

in a conventional op amp.

In this case, however, the positive input and negative output are

used for the feedback network. Because a conventional op amp

does not have a negative output, only its inverting input can be

used for the feedback network. The AD8132 is symmetrical, there-

fore, the feedback network on either side can be used to produce

the same results.

Because +IN is a summing junction, by an analogy to conven-

tional op amps, the gain from VIN to −OUT is −RF/RG. This holds

true regardless of the voltage on VOCM, and since +OUT moves

the same amount in the opposite direction from −OUT, the

overall gain is −2(RF/RG).

VOCM still governs VOUT,cm, so +OUT must be the only output that

moves when VOCM is varied. Because VOUT,cm is the average of the

two outputs, +OUT must move twice as far and in the same

direction as VOCM to create the proper VOUT, cm. Therefore, the

gain from VOCM to +OUT must be 2.

With β2 equal to 0 in these circuits, the gain can theoretically

be set to any value from close to 0 to infinity, just as it can with

a conventional op amp in the inverting mode. However, practical

real-world limitations and parasitics limit the range of acceptable

gain to more modest values.

β1 = 0

There is yet another class of circuits where there is no feed-

back from −OUT to +IN. This is the case where β1 = 0. The

differential amplifier without a resistor described in the

Differential Amplifier Without Resistors (High Input

Impedance Inverting Amplifier) section meets this condition, but

it was presented only with the condition that β2 = 1. Recall that

this circuit had a gain equal to 2.

If β2 decreases in this circuit from unity, a smaller part of +VOUT

is fed back to −IN and the gain increases (see Figure 68). This

circuit is very similar to a noninverting op amp configuration,

except for the presence of the additional complementary output.

Therefore, the overall gain is twice that of a noninverting op

amp or 2 × (1 + RF2/RG2) or 2 × (1/β2).

Once again, varying VOCM does not affect both outputs in the

same way; therefore, in addition to varying VOUT, cm with unity

gain, there is also an effect on VOUT, dm by changing VOCM.

ESTIMATING THE OUTPUT NOISE VOLTAGE

Similar to the case of a conventional op amp, the differential

output errors (noise and offset voltages) can be estimated by

multiplying the input referred terms, at +IN and −IN, by the

circuit noise gain. The noise gain is defined as

⎟

⎠

⎞

⎜

⎝

⎛

To compute the total output referred noise for the circuit of

Figure 64, consideration must be given to the contribution of

resistors, RF and RG. See Table 11 for estimated output noise

voltage densities at various closed-loop gains.

Table 11. Recommended Resistor Values and Noise

Performance for Specific Gains

Gain

(Ω)

Bandwidth

−3 dB (MHz)

Output

Noise

AD8132

Only

(nV/√Hz)

Output

Noise

AD8132

+ RG, RF

(nV/√Hz)

499

360

499

1.0 k

160

24.1

26.1

499

2.49 k

48.4

53.3

499

4.99 k

88.9

98.6

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