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THS6182 is unstable

Nicky Rosenberg
Prodigy 75 points

Hello,

I have a circuit which uses THS6182DWP. When turning the chip ON (Bias1, Bias2=0,0) there is an oscillation of 1.5 to 3Mhz on the output.

Here is the simplified circuit which behaves similar to the final one:

Vcc=24V

Gnd=0V

Ceramic cap of 2x22uF between Vcc and Gnd (pins 18 and 4,5,6,7,14,15,16,17)

(+) inputs (pins 8,13) are connected to Vcc/2

(-) inputs are connected via a resistor of 1k to the outputs: pin1-1k-pin2, pin20-1k-pin19

there is a load of 25R between the outputs pin 2 and 19.

When  (Bias1, Bias2=0,0)  turns the chip ON the oscillations appear, when the chip is OFF  (Bias1, Bias2=3.3V)  the oscillations disapear.

I can provide the detailed circuit, but only to the TI Engineer not in the public forum due to proprietary information.

Please advice ASAP

Thanks,

Nicky

  • 0
    TI__Genius 12631 points
    Nicky,
    There are a couple of things to try -
    1. Since this is a current feedback amplifier, the loop gain depends on the value of the feedback resistance (Rf) and the gain of the amplifier. Since you are using the amplifier in unity gain configuration you can try bumping Rf from 1k to around 1.5-2kOhm and see if the oscillation disappears.
    2. Try adding an extra bypass cap of 0.1uF-10nF as close to the DUTs power supply pins as possible. This could be the potential reason for the oscillation. 1.5-3MHz is much smaller than the amplifiers gain bandwidth product and oscillations at such low frequencies usually indicate a power supply bypass issue.

    If these don't fix the problem then there could also be issues with layout. Are you using the TI evaluation module (EVM)? If not, can you please send me a photograph of the board along with the layout? Also, when you are checking the amplifier for oscillations, please do not probe the output pin directly with a scope probe since that can add capacitance which may cause the amplifier to oscillate. The oscillation may not be there under normal circumstances and will only appear when probing the output. Add some isolation resistance between the scope probe and the amplifier output pin when doing this.

    -Samir