How to shield a battery powered instrumentation with USB interface?

Question

I would like to ask for advice on shielding the following setup.

I have a measurement target sitting inside a Faraday cage, connected through a shielded cable to a measurement circuit. The cable shield would be connected to the Faraday cage. The circuit mainly includes a microcontroller, AD converters and a single op-amp inverting transimpedance amplifier. The circuit sits inside a metallic box connected to the cable shield.

As the instrumentation needs to be battery powered (3.3V), the transimpedance amplifier is referred to the mid-level potential of 1.65V (wrt battery negative terminal) to allow measurement of current flow in both directions. This in turn means that at zero current flow, the measurement sample is at 1.65V wrt the negative terminal of the battery powering the measurement circuit. The analog signal bandwidth to be recorded is on the order of 10Hz-500Hz.

The device would occasionally have to be connected to a desktop computer through a USB interface. Then, the USB ground has to be connected to the negative terminal of the battery, 0V for the microcontroller and ADCs, -1.65V regarding zero transimpedance output.

I have received conflicting advice on how best to shield this circuit, one idea being to connect the shield to 1.65V, coupling the shield to the zero level of the transimpedance amplifier or alternatively, connecting the shield to the battery negative terminal, zero for the digital components.

In either case, a further question is whether the USB cable shield can or should be connected to the metal enclosure of the measurement circuit.

I have read a number of other posts on shielding, mainly as this, this and this, however I have not been able to piece these together for my case. Any help pointing me in the right direction would be much appreciated.

Answer 1

My advice: galvanically isolate it from the computer and USB.

While you haven't really described your problem, the fact you want a faraday cage implies you need it to be quiet, and anything connected to a computer never is.

You can isolate the digital interface quite easily.

If you are using I2C, then this is available galvanic isolated. For very low noise (in wafer probing RF testers), it has been used with a fibre connection

You will see that is isolates the serial (uart) interface to the USB (FT232). The coupler IC's are very easy to use. The only vice is they have capacitance across the barrier, and they also use UHF/Microwave RF signals. Neither are so much of an issue at your frequencies.

The TOSLINK fibre connection is great for very low noise: no metal enters the faraday shield, no cables come close, No RF is used, there is no capacitance. (and they are cheap and easy to get).

It is easy to feed uart over toslink - just hook the modules to your FTDI usb-serial board. The only drawback, is using a two wire serial (RX/TX only), so if you need handshaking, it has to be xon/xoff

[TOSLINK is the plastic optic fibre used by common audio equipment]