Precision Amplifier with Digital Control
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 and
DACT0009 are both useful in building instruments which have digital
control.
This circuit DACT0009 is similar to
DACT0008 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.
Vout = (Rf/Ri) * Vin
-o- Gain = Av = Rf/Ri
Digital Inputs - Logic 0 is 0V Logic 1 is 5V
|
X Y Value
|
B
|
A
|
Gain Av
|
0
|
0
|
0
|
1/10
|
1
|
0
|
1
|
1 |
2
|
1
|
0
|
10 |
3
|
1
|
1
|
100
|
|
|
