AD8237ARMZ-R7 datasheet(22/29 Pages) AD

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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

VOUT

G = 1 +

IB+

IB–

VREF

||R2

–

IBR

IBF

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

RS accomplishes this. If practical resistor values force the parallel

combination of R1 and R2 to be less than RS, add a series resistor

to the FB input to make up for the difference.

AD8237

+IN

–IN

REF

VOUT

VIN

RIN

IF R1||R2 = RS,

VOUT = VIN × (1 +

)

V+IN = VIN ×

RIN

RS + RIN

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.

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