Datasheet

AD8237 Data Sheet
Rev. 0 | Page 20 of 28
THEORY OF OPERATION
+IN
–IN
g
m1
I2
I1
I1 – I2
+
–
R2
R1
V
OUT
FB
REF
AD8237
g
m2
RFI
FILTER
TIA
+
–
+
–
RFI
FILTER
ALS
ALS
+
–
INTERNAL
IN-AMP
V
CM
=
V
S
2
V
CM
=
V
S
2
–IN
FB
TO g
m2
TO g
m1
+V
S
–V
S
+V
S
–V
S
RFI
FILTER
RFI
FILTER
+
–
+
–
+V
S
–V
S
+V
S
–V
S
10289-067
Figure 65. Simplified Schematic
ARCHITECTURE
The AD8237 is based on an indirect current feedback topology
consisting of three amplifiers: two matched transconductance
amplifiers that convert voltage to current, and one transimpedance
amplifier, TIA, that converts current to voltage.
To understand how the AD8237 works, first consider only the
internal in-amp. Assume a positive differential voltage is applied
across the inputs of the transconductance amplifier, g
m1
. This input
voltage is converted into a differential current, I1, by the g
m
.
Initially, I2 is zero; therefore, I1 is fed into the TIA, causing the
output to increase. If there is feedback from the output of the TIA
to the negative terminal of g
m2
, and the positive terminal is held
constant, the increasing output of the TIA causes I2, as shown, to
increase. When it is assumed that the TIA has infinite gain, the
loop is satisfied when I2 equals I1. Because the gain of g
m1
and g
m2
are
matched, this means that the differential input voltage across g
m
1
appears across the inputs of g
m2
. This behavioral model is all that
is needed for proper operation of the AD8237, and the rest of the
circuit is for performance optimization.
The AD8237 employs a novel adaptive level shift (ALS) technique.
This switched capacitor method shifts the common-mode level of
the input signal to the optimal level for the in-amp while preserving
the differential signal. Once this is accomplished, additional
performance benefits can be achieved by using the internal in-amp to
compare +IN to FB and −IN to REF. This is only practical because
the signals emitting from the ALS blocks are all referred to the
same common-mode potential.
In traditional instrumentation amplifiers, the input common-
mode voltage can limit the available output swing, typically depicted
in a hexagon plot of the input common-mode vs. the output voltage.
Because of this limit, very few instrumentation amplifiers can
measure small signals near either supply rail. Using the indirect
current feedback topology and ALS, the AD8237 achieves a truly
rail-to-rail characteristic. This increases power efficiency in many
applications by allowing for power supply reduction.
The AD8237 includes an RFI filter to remove high frequency out-
of-band signals without affecting input impedance and CMRR over
frequency. Additionally, there is a bandwidth mode pin to adjust the
compensation. For gains greater than or equal to 10, the bandwidth
mode pin (BW) can be tied to +V
S
to change the compensation
and increase the gain bandwidth product of the amplifier to 1 MHz.
Otherwise, connect BW to −V
S
for a 200 kHz gain bandwidth
product.
SETTING THE GAIN
There are several ways to configure the AD8237. The transfer
function of the AD8237 in the configuration in Figure 65 is
V
OUT
= G(V
+IN
− V
−IN
) + V
REF
where:
R1
R2
1 +=G
Table 7. Suggested Resistors for Various Gains (1% Resistors)
R1 (kΩ) R2 (kΩ) Gain
None Short 1.00
49.9 49.9 2.00
20 80.6 5.03
10 90.9 10.09
5 95.3 20.06
2 97.6 49.8
1 100 101
1 200 201
1
499
500
1 1000 1001
Whereas the ratio of R2 to R1 sets the gain, the designer determines
the absolute value of the resistors. Larger values reduce power
consumption and output loading; smaller values limit the FB input
bias current and input impedance errors. If the parallel combination
of R1 and R2 is greater than about 30 kΩ, the resistors start to
contribute to the noise. For best output swing and linearity, keep
(R1 + R2) || R
L
≥ 10 kΩ.