Analog Tips
single-ended output with an adjustable gain.
The gain of the system can be set by the ratio
of RF and RG1 with the assumption that RG2 =
RG1 and amplifier B has a gain of –1.
For example, the ADA4807-2, a 180 MHz dual
amplifier, can be built as an inverting amplifier
for this application and this circuit provides
lower noise. With a low quiescent current of
1000 μA per amplifier, the circuit is ideal for
low power, high resolution data conversion
systems.
The input common mode can go beyond the
supply voltages. The output is rail to rail, which
makes it useful in the presence of a large
common-mode signal or a large output voltage
applications. For example, the data acquisition
board has an ADC that accepts a single-ended
input of 0 V to 5 V. However, the signal source
happens to be differential voltage generated
from a sensor bridge where one terminal
swings positive while the other swings negative
in response to pressure in the presence of
common noise.
The plot in Figure 2 was taken by applying a
differential input voltage and varying the gain
of the circuit. The value of RF sets the gain
of the system. As one can observe, the plot
shows the system gain of 1, 2, and 4, with the
differential input voltage of 1 V p-p at 1 kHz.
The circuit is useful for measuring a small
difference between two large
voltages. For example, consider a
solution with a simple 1% accuracy
of monitoring a typical Wheatstone
bridge circuit that is excited by 3V/
GND in a 3V battery-powered system.
Using 1% resistors or better will allow
for the level of accuracy needed, and
the circuit will reject any common
mode and amplify the attenuated
bridge signal by the gain that the
circuit is set for. If driving an ADC,
some level-shifting will need to be
applied to get the output signal in the
range of 0V to 5V.
This circuit offers a combination of
excellent distortion with low quiescent
current. The 2two-op-amp solution
results in lower system cost and the
use of a differential amplifier yields
better performance.
Chau Tran joined Analog Devices in 1984,
where he works in the Instrumentation Amplifier
Products (IAP) group in Wilmington, MA. In
1990, he graduated with an MSEE degree
from Tufts University. Tran holds more than
10 patents and has authored more than 10
technical articles.
Figure 2. The performance of the simple differential to
single-ended amp
Jordyn Rombola is a product engineer in the
Linear and Precision Technology (LPT) Group
at ADI. She joined Analog Devices in January
2014 after finishing her bachelor’s degree
in electrical and computer engineering from
Worcester Polytechnic Institute (WPI).
21 DesignNews NOVEMBER 2017 www.eedesignnewseurope.com