The impedance problem

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ingram010

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Hi I need to read the voltage from a power supply, the datasheet for the power supply states that any digital measuring device needs an input impedance of at least 10M Ohms, unfortunately the Digital measuring device I have states that the maximum input impedance is 1M Ohm. What options do I have? Is there a circuit that can increase the input impedance of my measuring device?

Regards

John
 

A simple op-amp buffer would do the job if the voltage was acceptable but you would need a higher voltage to power it. Please tell us more abput the power supply, from what you describe it must have either very low current capability or has extremely sensitive current measuring built in to it.

Brian.
 


Hi Thanks for your reply

Here is the extract from the data sheet, it is a 1KV Glassman MK series.

Voltage monitor. J1-4
A 0-10v signal, positive with respect to common, and in direct proportion to output voltage, is available at this pin. a 10k Ohm limiting impedance protects the internal circuitry so that a digital voltmeter with greater than 10 Megohms input impedance should be used to monitor this output. It is also acceptable to use a 1 mA DC full scale instrument (i.e analog meter) for monitor purposes.

I am not using a multimeter to monitor the voltage I am using a labjack u3-HV.

I have a very tight budget so I am stuck with what I have already got.

Regards

John
 

What is the output impedance of the power supply? If it is less than 1 kohm the volt meter with input impedance of 1 Mohm will be good enough.
 

I do not have schematics for the PSU but if I'm right, what it is saying is a 100:1 potential divider allows you to monitor the output voltage with 10V FSD.. If a 1mA FSD meter can also be used, it implies the 'top' (1KV end) resistor is 990K and the bottom one is 10K. So a DVM with 1M input resistance would make a 10V reading show 9.902V which is an error of only ~1%. This may be within the tolerance of the DVM anyway. Do you really need it to be more accurate?

Brian.
 

No that is accurate enough. My main concern was that I might damage the labjack.

Regards

John
 

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