AD8131ARMZ datasheet(16/20 Pages) AD

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AD8131

Rev. B | Page 16 of 20

THEORY OF OPERATION

The AD8131 differs from conventional op amps in that it has

two outputs whose voltages move in opposite directions. Like

an op amp, it relies on high open-loop gain and negative

feedback to force these outputs to the desired voltages. The

AD8131 behaves much like a standard voltage feedback op amp

and makes it easy to perform single-ended-to-differential

conversion, common-mode level-shifting, and amplification of

differential signals.

Previous discrete and integrated differential driver designs used

two independent amplifiers and two independent feedback

loops, one to control each of the outputs. When these circuits

are driven from a single-ended source, the resulting outputs are

typically not well balanced. Achieving a balanced output

typically required exceptional matching of the amplifiers and

feedback networks.

DC common-mode level shifting has also been difficult with

previous differential drivers. Level shifting required the use of a

third amplifier and feedback loop to control the output

common-mode level. Sometimes the third amplifier has also

been used to attempt to correct an inherently unbalanced

circuit. Excellent performance over a wide frequency range has

proven difficult with this approach.

The AD8131 uses two feedback loops to separately control the

differential and common-mode output voltages. The differential

feedback, set by internal resistors, controls only the differential

output voltage. The common-mode feedback controls only the

common-mode output voltage. This architecture makes it easy

to arbitrarily set the common-mode output level. It is forced, by

internal common-mode feedback, to be equal to the voltage

applied to the VOCM input, without affecting the differential

output voltage.

The AD8131 architecture results in outputs that are very highly

balanced over a wide frequency range without requiring

external components or adjustments. The common-mode

feedback loop forces the signal component of the output

common-mode voltage to be zeroed. The result is nearly

perfectly balanced differential outputs, of identical amplitude

and exactly 180 degrees apart in phase.

ANALYZING AN APPLICATION CIRCUIT

The AD8131 uses high open-loop gain and negative feedback to

force its differential and common-mode output voltages in such

a way as to minimize the differential and common-mode error

voltages. The differential error voltage is defined as the voltage

between the differential inputs labeled +IN and −IN in

Figure 39. For most purposes, this voltage can be assumed to be

zero. Similarly, the difference between the actual output

common-mode voltage and the voltage applied to VOCM can also

be assumed to be zero. Starting from these two assumptions,

any application circuit can be analyzed.

CLOSED-LOOP GAIN

The differential mode gain of the circuit in Figure 39 can be

described by the following equation:

IN,

OUT,

where RF = 1.5 kΩ and RG = 750 Ω nominally.

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

⎟

⎠

⎞

⎜

⎝

⎛

The total output referred noise for the AD8131, including the

contributions of RF, RG, and op amp, is nominally 25 nV/√Hz

at 20 MHz.

CALCULATING THE INPUT IMPEDANCE OF AN

APPLICATION CIRCUIT

The effective input impedance of a circuit such as that in

Figure 39, 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

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

()

⎟⎟

⎟

⎠

⎞

⎜⎜

⎜

⎝

⎛

−

125

The 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.

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