AD8251ARMZ-R7 datasheet(19/24 Pages) AD

19 / 24 page

AD8251

Rev. 0 | Page 19 of 24

REFERENCE TERMINAL

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

(see Figure 49). 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 AD8251

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

AD8251

VREF

CORRECT

AD8251

OP1177

–

VREF

Figure 55. Driving the Reference Pin

COMMON-MODE INPUT VOLTAGE RANGE

The three op amp architecture of the AD8251 applies gain and

then removes the common-mode voltage. Therefore, internal

nodes in the AD8251 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 26 and

Figure 27 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

AD8251 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 AD8251 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 AD8251, follow these

guidelines:

•

Do not run digital lines under the device.

•

Run the analog ground plane under the AD8251.

•

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 AD8251 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 AD8251 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), 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 56. The

filter limits the input signal bandwidth according to the

following relationship:

)

(

FilterFreq

DIFF

FilterFreq

where CD ≥ 10 CC.

AD8251ARMZ-R7 Datasheet(PDF) 19 Page - Analog Devices

AD8251ARMZ-R7 Datasheet(HTML) 19 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL