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AD8428

Data Sheet

Rev. A | Page 18 of 20

APPLICATIONS INFORMATION

The classic 3-op-amp topology used for instrumentation

amplifiers typically places all the gain in the first stage and

subtracts the common-mode signals only in the second stage.

When operated at high gain, any amplifier is sensitive to large

interfering signals that can saturate it, thus making it impossible

to recover the signal of interest.

The AD8428 splits the total gain of 2000 into two stages: 200 in the

preamplification stage and 10 in the subtractor stage. Reducing the

gain of the first stage helps to increase the common-mode range

vs. differential signal range by avoiding saturation of the preamps.

–15

–10

–5

–15

–10

–5

OUTPUT VOLTAGE (V)

SINGLE STAGE GAIN, G = 2000

AD8428

Figure 46. AD8428 vs. Single Stage Gain Topology, G = 2000

In addition, filtering between stages can help to attenuate signals

before they reach the second amplification stage. This filtering

helps to prevent saturation of the second stage amplifier as long

as the signals are located in frequencies other than the signal of

interest.

EFFECT OF PASSIVE NETWORK ACROSS THE

FILTER TERMINALS

The AD8428 filter terminals allow access between the two

amplification stages. Adding a passive network between the two

terminals can shape the transfer function over the frequency of

the amplifier. The general expression for the transfer function is

represented by Equation 1.

6000

2000

Z(s)

G(s)

(1)

where Z(s) is the frequency dependent impedance of the

network across the filter terminals.

CIRCUITS USING THE FILTER TERMINALS

Setting the Amplifier to Different Gains

In its simplest form, the transfer function equation (Equation 1)

implies that the AD8428 can be configured for gains lower than

2000. This can be achieved by attaching a resistor across the filter

pins. Unlike the gain configuration of traditional instrumentation

amplifiers, this resistor attenuates the signal that was previously

amplified by the initial gain of 200.

Because this resistor appears inside the feedback of the subtractor

stage, it modifies the gain of the subtractor as well. The total gain

formula is a simplified version of the transfer function equation

(Equation 1).

6000

2000

(2)

The RG unit is in ohms. The resistor value required to obtain the

desired gain can be calculated using the following formula:

−

2000

6000

The AD8428 defaults to G = 2000 when no gain resistor is used.

When setting the amplifier to a different gain, the absolute gain

accuracy is only 10%. In addition, the temperature mismatch of

the external gain resistor increases the gain drift of the instrumen-

tation amplifier. Gain error and gain drift are at a guaranteed

minimum when a gain resistor is not used. For applications that

require accuracy at different gains, low noise, and wide bandwidth,

the AD8429 should be considered.

Low-Pass Filter

To help limit undesired differential signals, a first-order, low-pass

filter can be implemented by adding a capacitor across the filter

terminals of the AD8428, as shown in Figure 47.

+IN

–IN

–

AD8428

OUT

+FIL

–FIL

Figure 47. Differential Low-Pass Filter

This single-pole filter limits the signal bandwidth, as shown in

the following equation:

)

(

The 6 kΩ factor comes from the internal resistor values. The

tolerance of these resistors is 10%; therefore, using capacitors

with a tolerance better than 5% does not provide a significant

improvement on the absolute tolerance of the cutoff frequency.

Limiting the bandwidth of the amplifier also helps to minimize

the amount of out-of-band noise present at the output.

Note that filtering common-mode signals by adding a capacitor

on each filter terminal to ground degrades the performance of

the amplifier. This practice is generally discouraged because it

degrades CMRR performance. In addition, filtering common-

mode signals has little effect on preventing the saturation of the

internal nodes. On the contrary, the load added to the preamplifiers

causes them to saturate with even smaller common-mode signals.

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