AD8253ARMZ datasheet(19/24 Pages) AD

19 / 24 page

AD8253

Rev. 0 | Page 19 of 24

REFERENCE TERMINAL

The reference terminal, REF, is at one end of a 10 kΩ resistor

(see Figure 51). The instrumentation amplifier output is

referenced to the voltage on the REF terminal; this is useful

when the output signal needs to be offset to voltages other than

its local analog ground. For example, a voltage source can be

tied to the REF pin to level shift the output so that the AD8253

can interface with a single-supply ADC. The allowable reference

voltage range is a function of the gain, common-mode input,

and supply voltages. The REF pin should not exceed either +VS

or −VS by more than 0.5 V.

For best performance, especially in cases where the output is

not measured with respect to the REF terminal, source imped-

ance to the REF terminal should be kept low because parasitic

resistance can adversely affect CMRR and gain accuracy.

INCORRECT

AD8253

VREF

CORRECT

AD8253

OP1177

–

VREF

Figure 57. Driving the Reference Pin

COMMON-MODE INPUT VOLTAGE RANGE

The 3-op-amp architecture of the AD8253 applies gain and then

removes the common-mode voltage. Therefore, internal nodes

in the AD8253 experience a combination of both the gained

signal and the common-mode signal. This combined signal can

be limited by the voltage supplies even when the individual

input and output signals are not. Figure 28 and Figure 29 show

the allowable common-mode input voltage ranges for various

output voltages, supply voltages, and gains.

LAYOUT

Grounding

In mixed-signal circuits, low level analog signals need to be

isolated from the noisy digital environment. Designing with the

AD8253 is no exception. Its supply voltages are referenced to an

analog ground. Its digital circuit is referenced to a digital ground.

Although it is convenient to tie both grounds to a single ground

plane, the current traveling through the ground wires and PC

board can cause an error. Therefore, use separate analog and

digital ground planes. Only at one point, star ground, should

analog and digital ground meet.

The output voltage of the AD8253 develops with respect to the

potential on the reference terminal. Take care to tie REF to the

appropriate local analog ground or to connect it to a voltage that

is referenced to the local analog ground.

Coupling Noise

To prevent coupling noise onto the AD8253, follow these

guidelines:

•

Do not run digital lines under the device.

•

Run the analog ground plane under the AD8253.

•

Shield fast-switching signals with digital ground to avoid

radiating noise to other sections of the board, and never

run them near analog signal paths.

•

Avoid crossover of digital and analog signals.

•

Connect digital and analog ground at one point only

(typically under the ADC).

•

Power supply lines should use large traces to ensure a low

impedance path. Decoupling is necessary; follow the

guidelines listed in the Power Supply Regulation and

Bypassing section.

Common-Mode Rejection

The AD8253 has high CMRR over frequency, giving it greater

immunity to disturbances, such as line noise and its associated

harmonics, in contrast to typical in amps whose CMRR falls off

around 200 Hz. They often need common-mode filters at the

inputs to compensate for this shortcoming. The AD8253 is able

to reject CMRR over a greater frequency range, reducing the

need for input common-mode filtering.

Careful board layout maximizes system performance. To maintain

high CMRR over frequency, lay out the input traces symmetrically.

Ensure that the traces maintain resistive and capacitive balance;

this holds for additional PCB metal layers under the input pins

and traces. Source resistance and capacitance should be placed

as close to the inputs as possible. Should a trace cross the inputs

(from another layer), it should be routed perpendicular to the

input traces.

RF INTERFERENCE

RF rectification is often a problem when amplifiers are used in

applications where there are strong RF signals. The disturbance

can appear as a small dc offset voltage. High frequency signals

can be filtered with a low-pass RC network placed at the input

of the instrumentation amplifier, as shown in Figure 58. The

filter limits the input signal bandwidth according to the following

relationship:

)

(

FilterFreq

DIFF

FilterFreq

where CD ≥ 10 CC.

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