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Todd,

OPA124 is specified for the power supply range from +/5V to +/-18V (see table below); thus the minimum voltage for dual supply operation has to be Vcc= +/-5V while for a single supply operation minimum Vcc of 10V would be required.

The input common-mode voltage range is always a function of power supply voltage used - the PDS table below shows the typical common-mode input range of +/11V, and minimum of +/10V, at Vcc=+/-15V (see the conditions at the upper left corner of the PDS table below).  Therefore, the typical input voltage range, Vcm, is: (V-)+4V < Vcm < (V+)-4V while minimum guaranteed: (V-)+5V < Vcm < (V+)-5V.

This means that using Vcc=+/-5V, in order to assure proper operation, the input voltage of OPA124 must be zero: Vcm=0V.

Similarly, the output voltage range is also a function of the power supply voltage used; therefore, for Vcc=+/-5V, the typical OPA124 Vout=+/-2V while the minimum (guranteed) Vout=+/-1V.

This part is being used at +/-5VDC.  Let's say the positive supplied voltage dips to +4.5VDC - what performance impact should be expected?

Todd,

Since the OPA124 minimum total supply voltage is 10V (or +/-5V) lowering the positive supply to +4.5V would result in the total supply of 9.5V - this violates the minimum of voltage of 10V.  Since OPA124 is NOT tested under such condition, in such a case we may NOT guarantee any spec. 

However, since most of the specs will deteriorate gradually, you may expect to see significant increase in Vos (due to violation of PSRR, CMRR, and AOL minimum conditions), input offset drift as well as the decrease in the bandwidth and slew rate (see below).