MT-075 datasheet(3/9 Pages) AD

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MT-075

VOUT–

VOUT+

–

GAIN =

VIN+

VIN–

VOCM

i = 0

V+

V–

V+ = V–

VOCM

+ and – input currents are zero

+ and – input voltages are equal

Output voltages are 180° out of phase and symmetrical about VOCM

Gain = RF/RG

VOUT+ –VOUT–

VIN+ –VIN–

Figure 2: Analyzing Voltage Levels in Differential Amplifiers

Even if the external feedback networks (RF/RG) are mismatched, the internal common-mode

feedback loop will still force the outputs to remain balanced. The amplitudes of the signals at

each output will remain equal and 180° out of phase. The input-to-output differential-mode gain

will vary proportionately to the feedback mismatch, but the output balance will be unaffected.

Ratio matching errors in the external resistors will result in a degradation of the circuit's ability

to reject input common-mode signals, much the same as for a four-resistor difference amplifier

made from a conventional op amp.

Also, if the dc levels of the input and output common-mode voltages are different, matching

errors will result in a small differential-mode output offset voltage. For the G = 1 case with a

ground-referenced input signal and the output common-mode level set for 2.5 V, an output offset

of as much as 25 mV (1% of the difference in common-mode levels) can result if 1% tolerance

resistors are used. Resistors of 1% tolerance will result in a worst case input CMR of about 40

dB, worst case differential mode output offset of 25 mV due to 2.5 V level-shift, and no

significant degradation in output balance error.

The effective input impedance of a circuit, such as the one in Figure 2, at VIN+ and VIN– 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 ( VIN+

and VIN–) is simply:

RIN,dm = 2 × RG

Eq. 5

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