AD8132ARM datasheet(15/20 Pages) AD

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REV. 0

AD8132

–15–

Varying

While the circuit above sets

β2 to 1, another class of simple

circuits can be made that set

β2 equal to zero. 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 which can be analyzed separately (see Figure 8).

With –IN connected to ground, +IN becomes a “virtual ground”

in the same sense that the term is used in conventional op amps.

Both inputs must maintain the same voltage for equilibrium

operation, so if one is set to ground, the other will be 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. Since 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, so the

feedback network on either side can be used to produce the same

results.

Since +IN is a summing junction, by analogy to conventional op

amps, the gain from VIN to –OUT will be –RF/RG. This will hold

true regardless of the voltage on VOCM. And since +OUT will

move the same amount in the opposite direction from –OUT,

the overall gain will be –2 (RF/RG).

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

that moves when VOCM is varied. Since VOUT,cm is the average

of the two outputs, +OUT must move twice as fast and in the

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

the gain from VOCM to +OUT must be two.

In these circuits with

β2 equal to zero, the gain can theoretically

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

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

practical real-world limitations and parasitics will limit the range

of acceptable gains to more modest values.

1 = 0

There is yet another class of circuits where there is no feedback

from –OUT to +IN. This is the case where

β1 = 0. The resistorless

differential ampliﬁer described above meets this condition, but

it was presented only with the condition that

β2 = 1. Recall that

this circuit had a gain equal to two.

β2 is decreased in this circuit from unity, a smaller part of

+VOUT will be fed back to –IN and the gain will increase. See

Figure 5. This circuit is very similar to a noninverting op amp

conﬁguration, except for the presence of the additional comple-

mentary 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 will not affect both outputs in the

same way, so in addition to varying VOUT,cm with unity gain,

there will also be an affect 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 3, consideration must also be given to the contribution of

the resistors RF and RG. Refer to Table II for estimated output

noise voltage densities at various closed-loop gains.

Table II. Recommended Resistor Values and

Noise Performance for Speciﬁc Gains

Bandwidth Output Noise Output Noise

Gain

( )( )

–3 dB

AD8132 Only AD8132 + RG, RF

499 499

360 MHz

16 nV/

√Hz

17 nV/

√Hz

499 1.0 k

160 MHz

24.1 nV/

√Hz 26.1 nV/√Hz

499 2.49 k 65 MHz

48.4 nV/

√Hz 53.3 nV/√Hz

499 4.99 k 20 MHz

88.9 nV/

√Hz 98.6 nV/√Hz

Calculating an Application Circuit’s Input Impedance

The effective input impedance of a circuit such as that in Fig-

ure 3, at +DIN and –DIN, will depend on whether the ampliﬁer is

being driven by a single-ended or differential signal source. For

balanced differential input signals, the input impedance (RIN,dm)

between the inputs (+DIN and –DIN) is simply:

RIN,dm = 2

× R

In the case of a single-ended input signal (for example if –DIN is

grounded and the input signal is applied to +DIN), the input

impedance becomes:

IN dm

−

×+

()













The circuit’s input impedance is effectively higher than it would

be for a conventional op amp connected as an inverter because a

fraction of the differential output voltage appears at the inputs

as a common-mode signal, partially bootstrapping the voltage

across the input resistor RG.

Input Common-Mode Voltage Range in Single Supply

Applications

The AD8132 is optimized for level-shifting “ground” referenced

input signals. For a single-ended input this would imply, for

example, that the voltage at –DIN in Figure 3 would be zero

volts when the ampliﬁer’s negative power supply voltage (at V–)

was also set to zero volts.

Setting the Output Common-Mode Voltage

The AD8132’s VOCM pin is internally biased at a voltage approxi-

mately equal to the midsupply point (average value of the voltages

on V+ and V–). Relying on this internal bias will result in an

output common-mode voltage that is within about 100 mV of

the expected value.

In cases where more accurate control of the output common-mode

level is required, it is recommended that an external source,

or resistor divider (with RSOURCE < 10K), be used. The output

common-mode offset speciﬁed on pages 2 and 3 assume the

VOCM input is driven by a low impedance voltage source.

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