[sdiy] DMM problems [OT]

[Seb Francis]

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