The reason I'm asking this is that when I read about CMRR issue in amplifiers I only see differential amplifiers as if it is only a diff. amp. issue. But isn't a single ended amplifier also a differential amplifier where one input is zero with respect to the other input? Can we talk about CMRR issues when it comes to single ended inputs for a data acquisiton or it only applies to differential inputs?
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asked Nov 25, 2013 at 13:45
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3 Answers
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does cmrr value make sense for a differential amplifier if its inputs are separate and totally different voltages?
Yes, it makes sense. Consider two independent voltage sources as the inputs to a differential amplifier:
simulate this circuit – Schematic created using CircuitLab
This is equivalent to:
simulate this circuit Where we've defined the differential and common mode voltages to be:
$$V_d = V_{in1} - V_{in2}$$
$$V_{cm} = \dfrac{V_{in1} + V_{in2}}{2}$$
answered Nov 25, 2013 at 16:59
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\$\begingroup\$ u described it very clear way. now i understand what common voltage is about. \$\endgroup\$ Nov 25, 2013 at 17:36
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It only applies to differential amplifiers is the simple answer. But...
Both inputs of an op-amp will share the same common mode voltage with respect to signal ground (usually 0 volts). This voltage is normally 0 volts too so CMRR doesn't factor in but, some circuits will (for instance) produce a "fake" 0V with respect to the most negative voltage in the circuit and this may have noise on it relative to the most negative voltage. If this negative voltage is regarded as the AC signal ground (as in audio amps in quite a few cases) then CMRR should be taken into account AND also power supply rejection ration (PSRR).
answered Nov 25, 2013 at 14:03
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\$\begingroup\$ but not all differential amplifiers have to have the same input voltages. is that right? cmrr is measured when we apply the same input voltage to a differential amplifier. so does that mean cmrr makes sense only if we use a differential ended data acquisition where two inputs are exactly same? \$\endgroup\$ Nov 25, 2013 at 14:18
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The "C" in CMRR stands for "common". This refers to the signal that is the same on both inputs. In a single-ended input, there aren't two lines to have a common or not signal. The whole application of "common" to a single signal makes no sense. Common with what?
answered Nov 25, 2013 at 14:05