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Subject: Re: PCB problems please help

Date: 11/29/01 at 8:00 AM
Posted by: David Ashby
E-mail: dashby@quin.co.uk
Message Posted:

In Reply to: Re: PCB problems please help posted by Darren on 11/29/01 at 6:49 AM:

Hi Darren,

For a high impedance measurements, I can think of 3 options:

1st is simply to wire CMOS-input op-amps to the critical nodes of your circuit, configured as unity-gain followers. Just have them hanging in the air on the wires or upside down on your board. You then measure the voltage at the output of these op-amps without loading the critical node. The National Semiconductor LMC6061/6062/6064 op-amps have input bias currents of only a few pA. You'll have to allow for the input offset voltage of these devices, but for the LMC606X this will be under 1mV, which might be accurate enough for your purposes. Beware of input common mode range limitations: the LMC606X inputs can work close to GND, but not above positive supply minus 2.3V. Of course, you can extend the input swing by running these op-amps on a higher supply voltage than the 5V in the circuit - say 8 or 10V.

2nd: Linear Technology have published some high Z buffer circuits, one in application note AN9, and in Linear Technology Magazine Vol.3:issue 3. You'll be able to get the details from their website. This will give you high performance buffers, but will take some time to build & debug.

3rd: Hire a bench DVM with high input Z. Check the input Z spec - most DVMs switch from high impedance to 10Meg when a higher voltage range is selected, but if you go for a 6 1/2 digit Agilent(HP), Keithley, or Fluke, you'll should get high input Z up to 15 or 20V. The caveat here is input capacitance - although the input resistance is high, it is likely to be accompanied by a few nF!

I guess option 1 is easiest, unless the input offset voltage proves to be too much.


David Ashby

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