Issue with PGA when external reference is used in PSoC3/5

Question: In PGA configuration, when reference is selected as external and reference input is connected to internal available references i.e. 1.024V or 0.256V, output is not what is expected? How to fix it?

 

Answer:

In non inverting amplifier, reference in connected to input resistance. Reference generator should supply required current for the proper operation. You might have connected 1.024V reference to the reference input and could not make PGA work. The reason is, our references are not capable of driving current more than few 10’s of nA.

There are several ways to handle it:

 
1. Using Op-amp to buffer the reference: Op-amp can drive up to 25 mA current. So, it avoids the reference buffer getting overloaded. 
 
2. Using DSM reference buffer: DSM has internal reference buffer. Output of reference buffer can be routed to the output pin. To do it, simply ADC can be placed in schematic and in configuration (Figure 2), bypass the reference on output pin (Figure 3). But this solution can be used only when ADC is not being used for it’s intended functionality.
 
3. Using inverting PGA: This can be the best solution if it can be used in design instead of non inverting PGA.