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'Auto range'
1998\11\05@195300 by Justin Grimm

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Hi all
Im trying to build an autoranging circuit for a digital multimeter,
I was thinking of a pic controlling a voltage divider circuit, any ideas
on
how this can be done?

Thanks
Justin Grimm

1998\11\06@222436 by Chris Eddy

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I haven't done this yet, but am in the midst of a multimeter type project
just like yours.  I planned to simply setup trigger thresholds such that in
any given range, if the target threshold is exceeded for XXmS, then I switch
ranges.  Let the value settle, then try again.  I strongly suggest that you
look at the ceramic resistor elements available from Caddock.  I used one of
those and an Analog Devices analog mux part.  When I change ranges, I select
a different mux channel and run a different tap from the resistor element to
my ADC.  Wish I could tell you that it works, but I'm not there yet.  The
LCD display and '924 code displays bogus numbers, though!

Chris Eddy

Justin Grimm wrote:

> Hi all
> Im trying to build an autoranging circuit for a digital multimeter,
> I was thinking of a pic controlling a voltage divider circuit, any ideas
> on
> how this can be done?
>
> Thanks
> Justin Grimm

1998\11\07@142751 by Peter L. Peres

picon face
A small question: How do you handle input voltages that exceed Vdd or go
lower than GND with a tapped resistive divider connected to a multiplexer
?

I was thinking about using a high quality op-amp connected as inverting DC
amp. The input is into the inverting node through a fixed high quality
resistor and the gain is set by a multiplying DAC or resistor ladder and
mux. The reference voltage for ths amp is 1/2 Vref or 1/2 Vdd. This scheme
is applicable for differential input, allows input voltages to exceed Vdd
and go negative etc. The Ri is constant on all the scales. Do you see any
major disadvantage in this ?

Peter

1998\11\07@214315 by Russell McMahon

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I'm going to need to measure a signal with wide dynamic range but
modest accuracy for an upcoming project.
ie - the signal may vary by 10 bits or so but 8 bits of measurement
accuracy will suffice.

This is essentially the same as your requirement. One "nasty"
solution which also has some benefits is to take the single input and
divide it by various ratios and feed each output to a different A2D
input on an A2D equipped PIC. If a high sensitivity input is at full
range a higher division ratio input should be used. Conversely, if a
low sensitivity input has a low reading a higher sensitivity input
can be used.

Suitable care must be taken that the "over-ranging" of the high
sensitivity signals does not produce out of spec signals at the PIC.

I can imagine howls from the purists and it has limitations but this
method may be useful in some applications.



   Russell McMahon

From: Chris Eddy <spam_OUTceddyTakeThisOuTspamNB.NET>

>I haven't done this yet, but am in the midst of a multimeter type
project
>just like yours.  I planned to simply setup trigger thresholds such
that in
>any given range, if the target threshold is exceeded for XXmS, then
I switch
>ranges.  Let the value settle, then try again.  I strongly suggest
that you
>look at the ceramic resistor elements available from Caddock.  I
used one of
>those and an Analog Devices analog mux part.  When I change ranges,
I select
>a different mux channel and run a different tap from the resistor
element to
>my ADC.  Wish I could tell you that it works, but I'm not there yet.
The
>LCD display and '924 code displays bogus numbers, though!


AND

>Justin Grimm wrote:
>> Im trying to build an autoranging circuit for a digital
multimeter,
>> I was thinking of a pic controlling a voltage divider circuit, any
ideas
>> on
>> how this can be done?

1998\11\20@064506 by Justin Grimm

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face
Hi all
Im trying to build an autoranging circuit for a digital multimeter,
I was thinking of a pic controlling a voltage divider circuit, any ideas
on
how this can be done?

Thanks
Justin Grimm

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