Fully Differential Analog Attenuator
Abstract
Scientific instruments require high precision for
reliable measurements. For analog measurements
that go beyond the analog to digital converter voltage
threshold, an accurate voltage divider must be
constructed. The device constructed on a PCB uses
operational amplifiers and precision cut resistors to
attenuate the original signal into ADC device. The 20
channel board allows a division of 11 to 1 for the DAC
reading. The fully differential analog attenuator has a
maximum error of 1.05mV, which is acceptable for
precise measurements. For testing, the finished
product was used to measure voltages over lead acid
battery's and was successful. Measured errors on the
device never exceeded the predicted maximum. This
product is successful and will be used in navel
applications for pulsed power battery storage.
Christopher Williams
Electrical Engineering Department
The University of Texas of Arlington
Theory & Results
Voltage follower:
The voltage follower is used for multiple reasons. The most common is
to allow impedance matching for devices. The gain of a voltage follower
should be one, other wise known as unity gain. For a operational
amplifier to operate at unity gain the negative input should be used for
feedback to create stable operation. The results of using this voltage
follower was just as expected. The voltage inputted was the same
outputted and the NI 9204 data acquisition recorded all sampled data
correctly.[2]
Figure 4: LTSpice simulation of circuit model
Methods
Common methods used in the creation of a voltage
divider, biasing operational amplifier, and buffering
the signal. Simulating circuits with LTSpice to test
device components before final build.
Materials
Operational Amplifier- Analog Devices AD8479
Voltage Divider- Maxim MX5491
Voltage Follower- Texas Instruments OPA237
Figure 2: Voltage follower example circuit Vcc=12V,
Vin=Vout with a max of 15V. Right side photo shows
finished product. [1]
Differential Amplifier:
The differential amplifier is used to separate the original signal and
allow the resulting signal to be put into a voltage divider. The result of
our OPAMP circuit was successful but had an error of 1.03 mV. This was
acceptable due to error being small. The voltage divider follows this
circuit and consists of two laser cut resistors.
Figure 5: LTSpice simulation results of input voltage(red) to attenuated
voltage(blue). The results in the graphs show the input voltage was
reduced by a factor of 11 successfully. With upgraded components, that
handle higher voltages, the potential of this design could measure 2KV.
References:
[1] Cleveland Institute of Electronics. (2012, Jan) 1436
Voltage Follower Op Amp. PowerPoint.
[2] National Instruments Corporation. (2010) NI
CompactDAQ. Document.
Figure 1: Finished Fully Differential Analog
Attenuator Board Fully functional and in
current use.
Figure 3: Inputs are connected to a lead acid battery and then passed
to a voltage divider
Mentor: Matthew J. Martin, UTA PHD Student