AD8207
Rev. 0 | Page 10 of 16
THEORY OF OPERATION
The AD8207 is a single-supply, zero drift, difference amplifier
that uses a unique architecture to accurately amplify small
differential current shunt voltages in the presence of rapidly
changing common-mode voltage.
In typical applications, the AD8207 is used to measure current
by amplifying the voltage across a shunt resistor connected to
its inputs.
The AD8207 includes a zero-drift amplifier, a precision resistor
network, a common-mode control amplifier, and a precision
reference (see Figure 25).
A set of precision-trimmed resistors make up the network
that attenuates the input common-mode voltage to within the
supply range of the amplifier, in this case with a ratio of 20/1.
This attenuation ensures that when the input pins are externally
at the common-mode extremes of −4 V and +65 V, the actual
voltage at the inputs of the main amplifier is still within the
supply range.
The input resistor network also attenuates normal (differential)
mode voltages. Therefore, the total internal gain of the AD8207
is set to 400 V/V to provide a total system gain of 20 V/V.
Total Gain (V/V) = 1/20 (V/V) × 400 (V/V) = 20 V/V
The AD8207 is designed to provide excellent common-mode
rejection, even with PWM common-mode inputs that can
change at very fast rates, for example, 1 V/ns. An internal
common-mode control amplifier is used to maintain the input
common mode of the main amplifier at 3.5 V (with 5 V supply),
and therefore eliminates the negative effects of such fast-
changing external common-mode variations.
The AD8207 features an input offset drift of less than
500 nV/°C. This performance is achieved through a novel
zero-drift architecture that does not compromise band-
width, which is typically rated at 150 kHz.
The reference inputs, V
REF
1 and V
REF
2, are tied through 100 kΩ
resistors to the positive input of the main amplifier, which
allows the output offset to be adjusted anywhere in the output
operating range. The gain is 1 V/V from the reference pins to
the output when the reference pins are used in parallel. When
the pins are used to divide the supply, the gain is 0.5 V/V.
The AD8207 offers breakthrough performance without
compromising any of the robust application needs typical
of solenoid or motor control. The part rejects PWM input
common-mode voltages, while the zero-drift architecture yields
the lowest offset and offset drift performance on the market.
ZERO-DRIFT
AMPLIFIER
AD8207
COMMON-MODE
CONTROL AMPLIFIER
3.5V/2.2V
REF
60kΩ
120kΩ
SHUNT
120kΩ
+IN
GND
–IN
OUT
V
REF
1
V
REF
2
60kΩ
9kΩ
50kΩ
6kΩ 6kΩ
09160-025
100kΩ
100kΩ
100kΩ
100kΩ
100kΩ
Figure 25. Simplified Schematic
