Different grounds and differential signals

Discussion in 'General Electronics Chat' started by chaughwo, Jun 30, 2014.

  1. chaughwo

    Thread Starter New Member

    Jun 1, 2014
    I'm building a physics device called and MWPC which requires that a sense wire be held at a high potential (~1800 V) and that a small signal (~10 uA) be amplified from it through a TIA and then fed into an ADC as a differential pair. I've been trying to use a blocking capacitor to stop the 1800 V DC and pass the small transient signal to the amplifier and then use an AD8476 or similar IC to convert the single ended output from the amplifier to a differential signal for the ADC. Unfortunately, the blocking capacitor seems to be creating instabilities in the TIA circuit. My question is whether I could just set the ground on the TIA and the AD8476 to the 1800 V of the sense wire, have the power rails at an appropriate level relative to the "new" ground (~1795 V. and 1805 V.) and eliminate the blocking capacitor? Would the differential signal fry the ADC which is grounded at the "original" ground? I seem to recall the differential signals are not relative to any external voltage reference, but would the AD8476 isolate them well enough? What parameter on a data sheet would describe this isolation?

  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    I'd be surprised if you really need a TIA, assuming you are referring to a transimpedance amplifier.

    A 300Kohm resistor to ground will develop 3V at 10uA, while only perturbing your sense voltage by 3V/1800V, or .17%. This 3V signal can be buffered and driven directly to an ADC.

    Additional protection, if required, can be achieved using a series resistor into the buffer, along with appropriate clamping diodes.

    I do this with a high voltage corona measurement system, running at about 1700 volts, and it works well.