Precision Attenuator with Digital Control
The analog front end may carry high energy
components, protection and attenuation may be needed
to bring it to safe levels. After Signal
Conditioning measurement can be made.
The Circuits DACT0008 and DACT0009 are both useful
in building instruments which have digital control.
Precision Attenuator with digital control
The Circuit DACT0008 is
a programmable attenuator and the digital control
can be a remote dip switch, a CMOS Logic Output like
the A-B-C-D outputs of a decade counter, or an I/O
port of a uC like 80C31.
The heart of the circuit is the popular OP07 OpAmp
with Ultra Low Offset in the inverting
configuration, 4052 a CMOS analog multiplexer switch
enables the gain change, the innovation of the
circuit is that the on resistance ( around 100 ohms)
of 4052 switch is bypassed so that no error is
introduced by its use.
Circuit for Thermocouples
The resistors used R1 to R6 can be 0.1% 50ppm if you
will use a 3 ½ DPM i.e. + /- 1999 counts ( approx.
11 bit ), but for 4 ½ DPM ( approx. 14 bit ) you may
need to have trimpots2 in place of R3, R4, R5 &
R6 gain selection resistors to properly calibrate to
required accuracy but for testing or trials use 1%
100ppm MFR resistors but the errors will be around
To keep parts count (hence cost) to minimum the
common or ground is used as the positive input and
negative being one end of R1 this is because the
OpAmp inverts the polarity as it is used in
inverting configuration, this does not matter as the
equipment will be isolated by the power supply
transformer and all polarities are relative, but if
common has to be negative then add U4 and U5 as
shown in DACT0009.
Resistance LED Meter
The OP07 pin out is based on standard single OpAmp
741 and any other OpAmp like CA3140, TLO71, LF351
Can be used but with a lot off offset errors but for
trials any OpAmp may do but the errors may be >
1% and this is not tolerable n precision
instrumentation. OP07 has also equivalents
like uA714 & LM607 ultra low offset < 100uV
and low input bias <10nA and high input impedance
>100M are the key requirements for a good
instrumentation amp for DC inputs.
Vout = -(Rf/Ri) * Vin
-o- Gain = Av = Rf/Ri
|Digital Inputs - Logic 0 is
0V Logic 1 is 5V
|X Y Value
1 : A-B : A, B, C, D is 20 21
22 23 is 1, 2, 4, 8
2 : trimpots : e.g. replace R6 1K by 200E trimpot +
900E MFR. 900E can be from 1K parallel to 10K MFR.
a. Input 500 V max
1/4 W Resistor can withstand 250V D1 and D2 Clamps
the voltage to +/-0.5V thereby protecting OpAmp. R1
and R2 Limit the current also. D1 and D2
Clamps the voltage to +/-0.5V therby protecting
Output connect to DPM 7107/7135 or any other A/D
Convertor or OpAmp Stage. Use a buffer at output if
output has to be loaded by a value less than 1Meg.
Use an inverting buffer if input leads have to have
polarity where gnd is -In. See DACT0009 for details.
your own Meters and Instruments
c. 4052 CMOS Switch
The 4052/51/53 Analog Multiplexers have an on
Resistance of around 100E the highlight of the
circuit is that the CMOS on resistance comes in
series with the opamp output source resistance,
which produces no error at output.
Digital Control Options
A and B can be controlled by I/O port of uC, like
80C31 so that the uC can Control gain. A and B can
be given to Counters like 4029/4518 to scroll gain
digitally. A and B can be connected to DIP switch or
Circuit does not isolate only attenuates. When high
voltage is present at input any part of circuit is a
danger to touch.