Precision
Attenuator and Precision Amplifier
When Instruments are designed a analog front end is essential and also
as
most equipment have digital or microcontroller interface the analog
circuit
needs to have digital access. The Circuits DACT0008.SCH
and DACT0009.SCH are both useful in
building instruments which have digital control.
a. Precision Attenuator with digital control.
The
Circuit
DACT0008.SCH 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.
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 1%.
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.SCH.
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.
1 : A-B : A, B, C, D is 20 21 22 23
is 1, 2, 4, 8 respectively.
2 : trimpots : e.g. replace R6 1K by 200E trimpot + 900E MFR. 900E can
be from 1K parallel to 10K MFR.
b. Precision Amplifier with digital control.
This circuit is same as the DACT0008.SCH but gains of upto 100 can be
realized
in this configuration, this is useful for signal conditioning of low mV
outputs
of transducers. The gain selection resistors R3 to R6 can be selected
by
the user and can be anywhere from 1K to 1M and can also be trimpots for
obtaining
gains as required by user, the resistor values shown are for decade
gains
e.g. for an auto ranging DPM.
R1 and C1 reduce ripple in input and also snubs transients, ZD1 and ZD2
Zeners
clamp input to +/- 4.7V the input current is limited by R1 lastly C1
and
C2 are decoupling capacitors. The OpAmp U3 is used to increase the
input
impedance so that very low mV inputs are not loaded on measurement, the
user
can terminate the inputs with a resistor of his choice like 10M or 1M
to
avoid floating of the inputs when no measurement is being made. U5 is
used
as an Inverting buffer to restore polarity of the input and U4 is used
as
a buffer on the output of 4052 because loading it by resistance of
value
less than 1M will cause an error. An alternative is use R7 = R8 =1M and
remove
U4 but this may not be ideal. Gains of greeter than 100 may not be
practical
because at 100 gain itself a 100uV offset will be around 10mV at the
output
(100uV*100) this can be trimmed using the offset null option in the
OP07,
connect a trimpot between 1 and 8 and connect wiper to +5.
For better performance use ICL7650 ( not pin compatible ) instead of
OP07 and use +/- 7.5V instead of +/-5V supply.
Eight steps for gain or attenuation can be added by using two 4051 and
by
using Pin 6 Inhibit on 4051/52 limitless steps can be added by
cascading
many 4051,52,53 as Pin 6 works like a chip select.
Some extended applications of this circuits are....... Error correction
in
Transducer amplifiers by correcting gain. Auto ranging in DMM. Sensor
selection
or Input type selection in Process control. Digitally Preset power
supplies
or electronic loads. Programmable Precision mV or mA sources. PC or uC
or
uP based instruments. Data loggers and Scanners.