Re: Stable Micro volt source
- From: bill.sloman@xxxxxxxx
- Date: 9 Dec 2006 18:36:21 -0800
Syd wrote:
I only require 5 to10 voltages which I need to set to certain values within
the required range.
I have experimented with a simple bridge circuit using various values of
standard resistors (at this stage Not low temp coeff)
The output volage obtained is not very stable, although the entire circuit
was kept at the same temperature.
The problem is probably compounded by the need to have two pots in the
circuit to adjust the output volage.
I have never used a DAC before. I do not understand where the resistors are
connected. Can you suggest a DAC to use.
On sci.electronics.design it is normal practice to "bottom post" -
that is to add your response to the bottom of the message, rather than
sticking it on the front, as you have done here. I've copied your
response to the bottom of the post, and folowed with my reply.
<bill.sloman@xxxxxxxx> wrote in message
news:1165649780.936766.310260@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Syd wrote:
Can anyone help with a microprocessor controlled circuit to produce a
very
stable and settable -10 to +20 Microvolts supply which is temperature
stable. The voltages should be selectable and be able to step through a
series of selected microvolts.
How many discrete voltage do you want?
The easiest approach would probably be to use the microprocessor to
drive a DAC offering an output swing of +5V to -5V (or +2.5V to -2.5V)
- hook up a 143k (E96 0.1%) resistor to your DAC output, and a 432k
(E96 0.1%) resistor to your negative reference, and 2R (4R) resistor to
ground. Farnell stocks suitable parts.
The difficulty is avoiding microvolt thermocouple voltages between the
2 ohm resistor and the point where you want to see the 20uV - the only
easy option is to put the 2 ohm resistor at that point. You may then
have to be carefull to keep the resistance of your ground return path
well below 2 ohms, so the (up to) 10uA return current doesn't add to
the 20uV across the 2R resistor, but careful star grounding can get
around that.
Generating microvolts reliably does take a certain amount of thoughful
care, but it can be done.
The acid test of a microvolt generator is always to turn off or detach
your biasing source and see how many microvolts your measuring system
then sees - it should be zero, and hardly ever is .... a simple
mechanical switch to do the job can be handy, and something electronic
controlled by the microprocessor might let you do auto-offset
corection. A relay might not be a good idea - the coils get hot and can
generate thermocople voltages.
I only require 5 to10 voltages which I need to set to certain values within
the required range.
I have experimented with a simple bridge circuit using various values of
standard resistors (at this stage Not low temp coeff)
The output volage obtained is not very stable, although the entire circuit
was kept at the same temperature.
The problem is probably compounded by the need to have two pots in the
circuit to adjust the output volage.
I have never used a DAC before. I do not understand where the resistors are
connected. Can you suggest a DAC to use.
For this sort of applcation, you'd probably use one of the timing
counters inside the microprocessor to set up a pulse-width-modulated
(PWM) waveform. If you only require 5 to 10 specific values, an 8-bit
counter could do the job - you can usually get a 16-bit counter without
trying to hard.
You'd them feed the PWM waveform into the control pin of a multiplexer
- the 74HCT4052 would be nice
http://focus.ti.com/lit/ds/symlink/cd74hc4053.pdf
because the control voltage can be 0V to +5V while the switched outputs
can swing from +5V down to -5V.
One of the inputs to the multiplexer is your positive reference - the
LT1004-1.2 could be nice
http://www.linear.com/
and another would go to your negative reference, which you'd derive
from the +1.200V reference with an op amp and a pair of precision
resistors (or a precision divider). I'd go for -0.600V.
That would leave you two free inputs on a 4052, which I'd take to good
nearby ground connections, for the microprocessor to pick up when you
want to check if 0V reads 0V on your measuring device.
The output from the 4052 would then be fed into a a Sallen-Keys unity
gain low-pass filter to get rid of the PWM ripple and the output of the
filter would swing from -0.600V to +1.200V.
You'd get your -10uV to +20uV by dividing down this output much as
before - 120k into 2 ohms would do the job nicely. You've still got to
make sure that the 10uA running back to the power suplies doesn't shift
the "ground" at your measuring point by few uV - as little a an ohm
resistance in the return path could produce a 10uV drop along the
connection.
It can be important - one one occasion we managed to reverse the
polarity of an adjustable voltage reference because a dumb fault in the
printed circuit layout was running 100mA of digital supply current
through a sensitive analog ground connection. Easily fixed when we
worked out what was going on, but very embarassing in the meantime.
--
Bill Sloman, Nijmegen (but in Sydney at the moment).
.
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