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THS4524 vocm

sai prithvi
Prodigy 40 points
Other Parts Discussed in Thread: THS4524, ADS1278

hi...

I am using THS4524 as an amplifier on my analog board and a 0.9 v Vocm is required.

this Vocm is to be given externally. plz suggest the best possible ckt..

is using a resistor divider ckt advisable...

  • 0
    Prodigy 40 points

    is it possible to ground the vocm pin.....or will there be loading problem

     

  • 0 in reply to sai prithvi
    TI__Genius 17905 points

    Sai,

    It is possible to use a resistor divider, but there are several things to keep in mind. As discussed on p.25-26 of the datasheet in the section "Setting the Output Common-Mode Voltage," there is an internal resistor divider consisting of two 100kohm resistors that set the Vocm pin to mid-supply when the pin is left unconnected. When an external circuit is connected to the Vocm pin, it will have to supply or sink the current IEXT (given by Eqn 2 on p.26). If you decide to use an external resistor divider, the resistor ratio will be affected by IEXT Vocm pin. I've attached a quick TINA simulation that shows an external resistor divider that sets the Vocm pin voltage to 0.9V.

    0753.ths4524 vocm.TSC

    A couple potential issues with this approach is that any noise or variation in the supply used for the divider will be coupled into the Vocm voltage. The noise can be filtered out to some degree by placing a cap to ground on the Vocm pin (which I've included in the TINA schematic).

    I assume that the THS4524 will be driving an ADC. Many ADCs supply their required input common mode voltage as a reference output on a pin (called a variety of things, CM, VCM, REFOUT, etc). In this case, we usually recommend connecting that reference to the THS4524 Vocm pin, either directly if the ADC pin can directly supply/sink the current IEXT, or by using an op amp to buffer the reference voltage before driving the THS4524 Vocm pin (for an example, see datasheet Figure 81 on p.34 which shows a THS4524 + ADS1278 circuit).

  • 0 in reply to Kris F
    Prodigy 40 points

    Kristoffer,

    thank you for ur reply...

    I have tried your circuit and the Vocm isn't stable at 0.9v.It is fluctuating between 0.6v to 1.8v. In my circuit I have to use all 4 amplifiers parallely and hence i have shorted all the Vocm's.Does that effect the Vocm?

    Unfortunately I cant use the Vcm from the ADC. But I have tried to buffer the resistor divider output to the Vocm pins. But the end result remains the same.

    Sai

  • 0 in reply to sai prithvi
    Prodigy 40 points

    Also as per datasheet we are to observe Vocm to be 2.5v when left open.

    even that i observed isnt stable but fluctuating at some random voltages between(0.6-3.0).

  • 0 in reply to sai prithvi
    Genius 5430 points

    Sai;

    It sounds as if your amplifier is oscillating-- look at the output with a scope.

  • 0 in reply to Neil Albaugh
    TI__Genius 17905 points

    Sai,

    I agree with Neil that this sounds like an oscillation problem. Is this varying Vcm level occurring with no input signals applied? What loads are connected to the amplifier outputs? Along with looking at the output signals on an oscilloscope, I would also look at each of the THS4524 supply pins to make sure that the voltages at those pins are what you expect and are stable. As recommended in the datasheet, you should also have at least 0.22uF capacitors on each Vocm pin.

    Have you made sure that your feedback resistors for each amplifier connect the inverting input (-) to the Vout+ output and, on the other side, the non-inverting input (+) to the Vout- output?

    Can you share your schematic here? We may be able to find out why you are having this issue. You can attach it to a post here or you can send it to me through e-mail (k.flores@ti.com).