Integrated vs Discrete Programmable Gain Arrays

Hi Nikhil,

It is possible to build your own discrete PGA using a couple of op-amps, analog switches and precision resistors.  The gain error of the discrete PGA will be a direct function of the accuracy of the ratio of the resistors, and how well matched is the drift of the resistor ratios over temperature.  Therefore, the user will have to look for matched resistor pairs (or high precision resistors) depending on the gain error tolerance required in the application.  Also, the circuit designer has to be careful selecting the analog switches and the placement of the switches/muxes in the design, since the analog switches have an associated ON resistance that will add to the gain error.  Also the switch on resistance typically changes in non-linear fashion with input voltage.  In addition,  we also have to account for the analog switch parasitic capacitance for the stability and AC response of the design. 

That said, it is quite possible to build a simple discrete programmable amplifier using two op amps and a couple of analog switches, and sense the voltages with a second buffer amplifier.  The circuit below is a discrete PGA with 3 gain options.  The output voltage of the first stage is sensed right at the feedback resistor, with a high impedance buffer, eliminating in this fashion any errors of the switch ON resistance:

Integrated programmable gain amplifiers already incorporate well matched precision resistor networks inside the device  providing a low gain error specification.  Also, since they are integrated, they will tend to be lower area, and may add additional diagnostic features. for example  PGA281, PGA112, PGA113,...

However, as you have mentioned, a discrete design can always be customized by choosing the amplifiers and available gains specific to the application needs.

Than you and Regards,

Luis