AD8138ARZ-R7 datasheet(17/24 Pages) AD

17 / 24 page

AD8138

Rev. F | Page 17 of 24

THEORY OF OPERATION

The AD8138 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

AD8138 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. Also like an op amp, the AD8138 has high

input impedance and low output impedance.

Previous differential drivers, both discrete and integrated

designs, have been based on using 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 has 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 has 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 AD8138 uses two feedback loops to separately control the

differential and common-mode output voltages. The differential

feedback, set with external 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 output common-mode 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 AD8138 architecture results in outputs that are very highly

balanced over a wide frequency range without requiring tightly

matched external components. 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° apart in phase.

ANALYZING AN APPLICATION CIRCUIT

The AD8138 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 42.

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.

SETTING THE CLOSED-LOOP GAIN

Neglecting the capacitors CF, the differential-mode gain of the

circuit in Figure 42 can be determined to be described by

OUT

This assumes the input resistors,

RGS, and feedback resistors, RFS,

on each side are equal.

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 42, consideration must also be given to the contribution

of the Resistors RF and RG. Refer to Table 8 for the estimated

output noise voltage densities at various closed-loop gains.

Table 8.

Gain

(Ω)

Bandwidth

−3 dB

Output

Noise

AD8138

Only

Output

Noise

AD8138 +

RG, RF

499

320 MHz

10 nV/√Hz

11.6 nV/√Hz

499

1.0 k

180 MHz

15 nV/√Hz

18.2 nV/√Hz

499

2.49 k

70 MHz

30 nV/√Hz

37.9 nV/√Hz

499

4.99 k

30 MHz

55 nV/√Hz

70.8 nV/√Hz

AD8138ARZ-R7 Datasheet(PDF) 17 Page - Analog Devices

AD8138ARZ-R7 Datasheet(HTML) 17 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL