Ground current and amplifier

[theredkid]

Dear All,

I have 2 questions that I need to solve and hand in. Could you pls help me with the following?

1) When using high voltage supplies, safety requirements dictate that a safety ground has to be connected between the HV supply ground terminal and a building ground. However, this creates a ground loop, with ground currents flowing through ground screens in the signal cables between the control unit and the HV supply. How can these ground currents be minimized?

2) Sketch an amplifier circuit for measuring currents up to 1 μA from DC to 1 kHz. Input impedance shall be lower than 100 Ω. Output voltage at 1 μA shall be 1 V.

Thank you/Red

 

[HTA]

Safety ground is a protective ground for humans touching the case or signal line. In this view I think the need is to protect the HV supply from being touched by a surrpounding metal cage and connect protective ground there. If this is not possible you could do an isolation of the incomming control signals.

Enjoy your design work!

 

[jiripolivka]

For safety grounding connect the safety and signal grounds to one point. Then there will be no ground loop.

To amplify the signal voltage at low fequency use an operational amplifier. If the source impedance is 100 Ohms, one uA through it generates 100 uV. To get one volt output you need a 10 000 gain. A good choice is an "instrumentation" amplifier that can operate from DC to 1 kHz. Check "www.analog.com", there are many such opamps including recommended schematics.

 

[godfreyl]

Regarding the current measurement question:
Since the maximum input voltage is less than 100uV, your main problems are going to be DC offset and noise. You may want to consider a chopper-stabilised amp for that.