AD8250

Data Sheet

Rev. C | Page 18 of 24

REFERENCE TERMINAL

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

(see Figure 47). 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 AD8250 can interface

with a single-supply ADC. The allowable reference voltage

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

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

AD8250

CORRECT

AD8250

OP1177

+

–

Figure 53. Driving the Reference Pin

COMMON-MODE INPUT VOLTAGE RANGE

The 3-op-amp architecture of the AD8250 applies gain and then

removes the common-mode voltage. Therefore, internal nodes

in the AD8250 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 27 and Figure 28 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

AD8250 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 PCB

can cause errors. Therefore, use separate analog and digital ground

planes. Analog and digital ground should meet at only one point:

star ground.

The output voltage of the AD8250 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 AD8250, do the following

guidelines:

•

Do not run digital lines under the device.

•

Run the analog ground plane under the AD8250.

•

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).

•

Use the large traces on power supply lines 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 AD8250 has high CMRR over frequency, giving it greater

immunity to disturbances, such as line noise and its associated

harmonics, in contrast to typical instrumentation amplifiers

whose CMRR falls off around 200 Hz. Typical instrumentation

amplifiers often need common-mode filters at their inputs to

compensate for this shortcoming. The AD8250 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 capaci-

tance should be placed as close to the inputs as possible. Should a

trace cross the inputs (from another layer), route it perpendicular

to the input traces.