AD8237ARMZ-R7 datasheet(21/29 Pages) AD

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AD8237

Data Sheet

Rev. 0 | Page 20 of 28

THEORY OF OPERATION

+IN

–IN

gm1

I1 – I2

VOUT

REF

AD8237

gm2

RFI

FILTER

TIA

–

RFI

FILTER

ALS

INTERNAL

IN-AMP

VCM =

–IN

TO gm2

TO gm1

+VS

–VS

+VS

–VS

RFI

FILTER

RFI

FILTER

–

+VS

–VS

+VS

–VS

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, gm1. This input

voltage is converted into a differential current, I1, by the gm.

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 gm2, 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 gm1 and gm2 are

matched, this means that the differential input voltage across gm1

appears across the inputs of gm2. 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 +VS to change the compensation

and increase the gain bandwidth product of the amplifier to 1 MHz.

Otherwise, connect BW to −VS 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

VOUT = G(V+IN − V−IN) + VREF

where:

1 +

Table 7. Suggested Resistors for Various Gains (1% Resistors)

R1 (kΩ)

R2 (kΩ)

Gain

None

Short

1.00

49.9

2.00

80.6

5.03

90.9

10.09

95.3

20.06

97.6

49.8

100

101

200

201

499

500

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) || RL ≥ 10 kΩ.

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