From: Tony Williams
Subject: Re: Voltage Controlled Resistor?
Date: Fri, 13 Dec 2002 23:32:15 +0000 (GMT)
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NNTP-Posting-Date: Fri, 13 Dec 2002 23:32:54 +0000 (UTC)
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In article ,
Winfield Hill wrote:
> . Rin = 40k Vset/10
> . IN o-------------------------,
> . G =_+1 |
> . __ R1 ___/ |___| ____ R3 ___ Vref = 10V
> . | 40k \_| | / 40k
> . | ____ | __/
> . | | | ____ R2___ Vset
> . | | | | 40k 0 to +10V
> . o x o y o out o z
> . _______| | | |_______
> . | | | | | |
> . | |/ Q1 | | Q4 \| |
> . gnd --|-----| ______| | |-----|-- gnd
> . | |\V | | | V/| |
> . | | | Q2 |/ Q3 \| | |
> . | _ +--|----| |-----+ _ |
> . '--|-\__| | |\V V/| |__/-|--'
> . V1 ---|+/ | _ |_____| \+|--- gnd
> . '--|-\____|
> . V2 ---|+/ Iout = Ix Iy / Iz
Just to clarify, everything below the x,y,out,z line
is inside the RC4200.
> Oops, I see the issue, you need higher output currents and a
> 1k rather than 40k range. I see, the extra opamp FET and
> shunt are so you can keep the currents below 250uA.
Yes. Note also that the Q3 collector output is limited
to 5V above the 0v pin.
> I wonder how much effort is required to get all the offsets
> right, etc., for 0.1% performance (fig 7).
The Fig.7 in my data sheet is marked "Extended Range Divider
and the large collection of resistors around the RC4200
is to do with obtaining a bipolar Vx and Vout.
As noted in another post, this particular topology does
require taking the reciprocal of Vset (as per the use of
the z input in your circuit above). This would raise
concerns about the Vos of the z input when operating with
low values of Vset.
Note though that the z input opamp does not have a Vos
> AD734, wink wink. Anyway, very nice, I had forgotten that
> part. Great that it's still available,
I've used the RC4200 twice on jobs. Worked ok both times,
no nasty surprises, all devices well within the tyical spec.
Trouble is it's one of those chips that grabs your interest,
but the minor limitations/inconveniences reduces the number
of job you can actually use it on.