[sdiy] DMM problems [OT]

[Seb Francis]

No, I meant the 1M resistor is part of the DMM circuit itself.  The 
signal for the <400mV range is input to the MAX134 via a 1M resistor.  
The other volts ranges are input via a 10M resistor to a different pin 
on the MAX134.  Since it is only the mV reading that is wrong (and the 
Ohms readings which also use this same input pin on the MAX134 as the mV 
range) I assume it must be an internal fault of the MAX134 (maybe one of 
the internal analog switches?)  I could be wrong, but swapping the chip 
out will confirm it.

Seb


jays at aracnet.com wrote:
> Seb,
>
> I don't think I'd trust the 1 Meg ohm reading. That is a really high resistor value (pretty close to an open circuit) and the reading could just be just background noise. Depends on the scaling and sensitivity of the circuit. If you have a scope with a 1 Meg ohm probe put your finger on it and see what you get. You might be surprised. Or just put your finger on the + lead and see what happens.
>
> Jay S.
>
>
> Seb Francis wrote:
>
>   
>> Final update..
>>
>> I think I've now tracked the problem down to definitely being the MAX134 
>> chip.  I noticed that also the mV mode was reading 23mV with the inputs 
>> shorted.  The mV mode and the Ohms mode have in common that they both 
>> use a different input pin on the MAX134 chip than the higher voltage 
>> modes.  And the mV input circuitry is very simple, so I can't see 
>> anything else that could cause the problem: Input is via PTC protection 
>> resistor to 1M resistor to '400mV' input of MAX134.
>>
>> If I ground the 400mV input directly then it reads 0, but when grounded 
>> via the 1M resistor it reads 23mV, so I'm assuming that something is not 
>> quite right inside the MAX134 that the 1M resistor should make that much 
>> different (the input bias current is spec'ed to be only 5pA).
>>
>> So hopefully the nice people at Maxim will send me a new chip, then I'll 
>> have to see how I get on with my first ever SM soldering experience ... 
>> replacing a 44 pin MQFP IC!
>>
>> Seb
>>
>>     
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