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DA18000AC Differential Amplifier
Voltage on +
input to ground
input to ground
+1.5 V
-1.5 V
+0.8 V
+1.0 V
+6.5 V
1.5 V
sine
pk-pk
Some amplitudes are specified as "peak to peak." The differential amplifier peak-to-peak
range will be twice the peak differential mode range specification as, at any instant in time,
the maximum voltage amplitude signal would be one-half of the peak-to-peak value.
In a balanced differential system, the signal on each output will be an inverted copy of the
other input. For example, an LVDS system may have a pair of outputs, each of which has a
voltage swing of 0 to +370 mV. A logic 1 would be represented when the + output is at
+370 mV, while the - output is at 0 V. A logic zero would be the opposite polarity: the + output
at 0 V and the - output at +370 mV. Note that even though both outputs swing 370 mV, the
maximum difference voltage between them at any instant is still within ±370 mV. So this sig-
nal could be measured with a differential amplifier that has a differential mode range of ±400
mV.
COMMON MODE REJECTION RATIO
The ideal differential amplifier/amplifier would sense and amplify only the differential mode
voltage component and reject all of the common mode voltage component. Real differential
amplifiers are not perfect and a small portion of the common mode voltage component
appears at the output. Common Mode Rejection Ratio (CMRR) is the measure of how much
the amplifier rejects the common mode voltage component. CMRR is equal to the differential
mode gain (or normal gain) divided by the common mode gain. Common mode gain is equal
to the output voltage divided by the input voltage when both inputs are driven by only the
common mode signal. CMRR can be expressed as a ratio (e.g. 10,000:1) or implicitly in dB
(e.g. 80 dB). Higher numbers indicate greater rejection (better performance).
The first order term which determines the CMRR is the relative gain matching between the +
and – input paths. To obtain high CMRR values, the input attenuators in a differential ampli-
fier are precisely matched to each other. The matching includes the DC attenuation as well
as the capacitance which determines the AC attenuation. As the frequency of the common
6-2
Voltage on -
Difference
+0.8 V
+0.7 V
-0.8 V
-0.7 V
-0.1V
+0.9 V
-1.0 V
+2.0 V
+6.0 V
0.5 V
Ground
0.75 Vpeak
Comment
OK: within ±1 V
range
OK: within ±1 V
range
OK: within ±1 V
range
Out of range:
exceeds ±1 V
Exceeds ±5 V
common mode
range
OK: within ±1 V
range
DA18000AC-OM-E Rev A
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