Programmable Gain Amplifier with Vdda/2 as reference

[Anonymous]

I've come across a perplexing problem. I have a CY8CKIT-059 and using PSOC Creator 4.3. I have a Programmable Gain Amplifier (PGA) with a gain of 2, and the reference set to external, and attached to a Vref component. With the Vref component set to Vdda/2, the reference voltage will actually change when I change the input voltage to the PGA. When the input votlage is zero, the voltage from the VREF component will read 1.4 V, and it increases tp 3.4 V when the PGA input is near 5V. This does not happen with any of the other reference voltage options (1.024 V, Vdda etc). Does anyone have any insights into this problem?

[odissey1 · Accepted solution]

MiRa,

The Vdda/2 impedance is 50k (100k+100k), while PGA input impedance in ~20k. So PGA input actually heavily loading VRef. The VRef=1,024V is internally buffered, so loading is not that terrible. You can buffer VRef=Vdda/2 using internal Opamp in follower mode, or use external Vdda/2 using 2x10k resistors.

/odissey1

[Anonymous]

Yes, this is exactly what I was looking for. Thank you!

[Anonymous]

By the way, why is the PGA input impedance so low?

[odissey1]

Take a look on the values of gain setting resistors in the datasheet.

[Len_CONSULTRON]

MiRa,

If you connect to a op-amp, the input impedance can be very high (>>1M ohm).  A PGA is an op-amp with additional input resistance (Rbias) to GND or VDDA and an output (Rfb through a feedback resistor).  The AC resistance equivalent (impedance) is the parallel resistances of Rfb | Rbias = Rimp which is always lower than the resistance of either resistor.  Therefore for example, if Rbias = 50K and Rfb = 50K, Rimp = 25K.  With these resistances, the gain is x2.  [ie: (Rfb/Rbias) + 1 = (50K/50K) + 1  = 2].

Len