ad8251nbsp Analog Devices, Inc., ad8251nbsp Datasheet - Page 19

• Manufacturer Part Number: ad8251nbsp
• Description: 10 Mhz, 20 V/?s, G = 1, 2, 4, 8 I Cmos Programmable Gain Instrumentation Amplifier
• Manufacturer: Analog Devices, Inc.

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 +V
or −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.
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.
S
by more than 0.5 V.
V
REF
INCORRECT
AD8251
Figure 55. Driving the Reference Pin
V
REF
+
OP1177
–
CORRECT
AD8251
Rev. 0 | Page 19 of 24
S
Coupling Noise
To prevent coupling noise onto the AD8251, follow these
guidelines:
•
•
•
•
•
•
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:
where C
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.
FilterFreq
FilterFreq
D
≥ 10 C
CM
DIFF
C
.
=
=
2
2
π
π
1
RC
( R
2
C
C
1
D
+
C
C
)
AD8251