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FDA - Selectable Gain implementation

Peter Baxter
Intellectual 380 points
Other Parts Discussed in Thread: PGA870, TS5A21366, THS4521

What is the proper way to add selectable gain to a FDA?

I wish to cascade two FDA sections to get gains of: x1, x2, x5, x10 in each stage (think Oscilloscope).

Multiple attenuation networks based around Rg - appear to be the most obvious solution - unless there are traps to be aware of.   

 

  • 0
    TI__Genius 17905 points

    Hi Peter,

    We do have the PGA870, which is a digitally programmable gain FDA. Please take a look and see if it meets your needs.

    With regards to implementing programmable gain with a regular FDA, it should be possible to use a series of switches and resistors. It is important to avoid adding excess capacitance (from the switches) to the summing junctions of the FDA (i.e. the non-inverting and inverting input pins) and directly at the FDA output. Capacitance at these nodes can lead to frequency response peaking and high frequency ringing/oscillations.

    Please see the attached TINA simulation using the THS4521. I cascaded two THS4521 FDAs, though I only implemented the resistor divider switch network in the second stage. I used ideal switches, but I set the On-resistance and added capacitors on the sides of each switch using values from the TS5A21366 datasheet. The TS5A21366 is a quad single-pole single-throw switch IC. Note that the switch capacitances are isolated with resistors from the sensitive amplifier inputs and outputs.

    I have not tested such a configuration, but see no reason why it would not work. There are a couple of things to keep in mind. Gain accuracy will depend on many more resistors (including switch ON-resistance) than a typical op amp circuit. Also, the switch capacitances will play a role in the bandwidth. This is not particularly reflected in my example with the THS4521, which only has a typical bandwidth of 9.3MHz in a gain of 10V/V configuration. With higher speed FDAs, the RC filters formed by the signal path resistors and the switch capacitances will limit the signal bandwidth.

    THS4521vargain.tsc