Datasheet

Data Sheet AD8237
Rev. 0 | Page 21 of 28
The bias current at the FB pin is dependent on the common-mode
and differential input impedance. FB bias current errors from the
common-mode input impedance can be reduced by placing a resistor
value of R1||R2 in series with the REF terminal, as shown in
Figure 66. At higher gains, this resistor can simply be the same
value as R1.
AD8237
+IN
–IN
REF
FB
V
OUT
G = 1 +
R2
R1
I
B
+
I
B
–
V
REF
R1
R2
R1
||R2
+
–
I
B
R
I
B
F
10289-068
Figure 66. Cancelling Error from FB Input Bias Current
Some applications may be able to take advantage of the symmetry of
the input transconductance amplifiers by canceling the differential
input impedance errors, as shown in Figure 67. If the source resistance
is well known, setting the parallel combination of R1 and R2 equal to
R
S
accomplishes this. If practical resistor values force the parallel
combination of R1 and R2 to be less than R
S
, add a series resistor
to the FB input to make up for the difference.
AD8237
+IN
–IN
REF
FB
V
OUT
R1 R2
V
IN
R
S
R
IN
R
IN
IF R1||R2 = R
S
,
V
OUT
= V
IN
× (1 +
R2
R1
)
V
+IN
= V
IN
×
R
IN
R
S
+ R
IN
10289-069
Figure 67. Canceling Input Impedance Errors
GAIN ACCURACY
Unlike most instrumentation amplifiers, the relative match of the
two gain setting resistors determines the gain accuracy of the AD8237
rather than a single external resistor. For example, if two resistors have
exactly the same absolute error, there is no error in gain. Conversely,
two 1% resistors can cause approximately 2% maximum gain error
at high gains. Temperature coefficient mismatch of the gain setting
resistors increases the gain drift of the instrumentation amplifier
circuit according to the gain equation. Because these external resistors
do not have to match any on-chip resistors, resistors with good TCR
tracking can achieve excellent gain drift without the need for a low
absolute TCR.
For the best performance, keep the two input pairs (+IN and −IN,
and FB and REF) at similar dc and ac common-mode potentials. This
has two benefits. For dc common-mode, this minimizes the gain
error of the AD8237. For ac common-mode, this yields improved
frequency response. There is a maximum rate at which the ALS
circuit can shift the common-mode voltage, which is shown in
Figure 27. Because of this limit, the best large signal frequency
response is achieved when the ac common-mode voltage of the two
input pairs are matched. For example, if the negative input is at
a fixed voltage and the positive input is driven with a signal, the
feedback input moves with the positive input; therefore, the ac
common-mode voltage of the two input pairs is the same. The
effect of this is shown in Figure 25 and Figure 26.
CLOCK FEEDTHROUGH
The AD8237 uses nonoverlapping clocks to perform the chopping
and ALS functions. The input voltage-to-current amplifiers are
chopped at approximately 27 kHz.
Although there is internal ripple-suppression circuitry, trace
amounts of these clock frequencies and their harmonics can be
observed at the output in some configurations. These ripples are
typically 100 µV RTI when the bandwidth is greater than the clock
frequency. They can be larger after a transient pulse but settle back
to nominal, which is included in the settling time specifications.
The amount of feedthrough at the output is dependent upon the
gain and bandwidth mode. The worst case is in high bandwidth
mode when the gain can be almost 40 before the clock ripple is
outside the bandwidth of the amplifier. For some applications, it
may be necessary to use additional filtering after the AD8237 to
remove this ripple.
INPUT VOLTAGE RANGE
The allowable input range of the AD8237 is much simpler than
traditional architectures. For the transfer function of the AD8237 to
be valid, the input voltage must follow two rules
• Keep the differential input voltage within the limits shown in
Figure 14; approximately ±(Total Supply Voltage – 1.2) V.
• Keep the voltage of the inputs (including the REF and FB pins)
and the output within the specified voltage range, which are
approximately the supply rails.
Because the output swing is completely independent of the input
common-mode voltage, there are no hexagonal figures or complicated
formulas to follow, and no limitation for the output swing the
amplifier has for input signals with changing common mode.