AD629A AD [Analog Devices], AD629A Datasheet - Page 8

• Manufacturer Part Number: AD629A
• Description: High Common-Mode Voltage Difference Amplifier
• Manufacturer: AD [Analog Devices]

AD629
APPLICATIONS
Basic Connections
Figure 29 shows the basic connections for operating the AD629
with a dual supply. A supply voltage of between ± 3 V and
± 18 V is applied between Pins 7 and 4. Both supplies should be
decoupled close to the pins using 0.1 µF capacitors. 10 µF elec-
trolytic capacitors, also located close to the supply pins, may
also be required if low frequency noise is present on the power
supply. While multiple amplifiers can be decoupled by a single
set of 10 µF capacitors, each in amp should have its own set of
0.1 µF capacitors so that the decoupling point can be located
physically close to the power pins.
The differential input signal, which will typically result from a
load current flowing through a small shunt resistor, is applied to
Pins 2 and 3 with the polarity shown in order to obtain a posi-
tive gain. The common-mode range on the differential input
signal can range from –270 V to +270 V and the maximum dif-
ferential range is ± 13 V. When configured as shown, the device
operates as a simple gain-of-one differential-to-single-ended
amplifier, the output voltage being the shunt resistance times the
shunt current. The output is measured with respect to Pins 1 and 5.
Pins 1 and 5 (REF(–) and REF(+)) should be grounded for a
gain of unity and should be connected to the same low imped-
ance ground plane. Failure to do this will result in degraded
common-mode rejection. Pin 8 is a no connect pin and should
be left open.
Single Supply Operation
Figure 30 shows the connections for operating the AD629 with
a single supply. Because the output can swing to within only
about 2 V of either rail, it is necessary to apply an offset to the
output. This can be conveniently done by connecting REF(+) and
REF(–) to a low impedance reference voltage (some analog-
to-digital converters provide this voltage as an output), which is
capable of sinking current. Thus, for a single supply of 10 V,
V
allow the output to swing ± 3 V around the central 5 V reference
voltage. Alternatively, for unipolar input signals, V
set to about 2 V, allowing the output to swing from +2 V (for a 0 V
input) to within 2 V of the positive rail.
I
SHUNT
REF
TEXT)
(SEE
might be set to 5 V for a bipolar input signal. This would
R
SHUNT
0.1 F
–3V TO –18V
REF(–)
Figure 29. Basic Connections
–V
–V
–IN
+IN
S
S
1
2
3
4
NC = NO CONNECT
380k
21.1k
380k
AD629
380k
20k
8
7
6
5
NC
REF(+)
+V
3V TO 18V
S
+V
S
0.1 F
V
OUT
= I
SHUNT
REF
(SEE
TEXT)
could be
R
SHUNT
–8–
Applying a reference voltage to REF(+) and REF(–) and operating
on a single supply will reduce the input common-mode range of
the AD629. The new input common-mode range depends upon
the voltage at the inverting and noninverting inputs of the internal
operational amplifier, labeled V
nodes can swing to within 1 V of either rail. So for a (single)
supply voltage of 10 V, V
9 V. If V
is +85 V to –75 V. The common-mode voltage ranges can be
calculated using the following equation.
System-Level Decoupling and Grounding
The use of ground planes is recommended to minimize the
impedance of ground returns (and hence the size of dc errors).
Figure 31 shows how to work with grounding in a mixed-signal
environment, that is, with digital and analog signals present. In
order to isolate low-level analog signals from a noisy digital
environment, many data-acquisition components have separate
analog and digital ground returns. All ground pins from mixed-
signal components such as analog-to-digital converters should
be returned through the “high quality” analog ground plane.
This includes the digital ground lines of mixed-signal converters
that should also be connected to the analog ground plane. This
may seem to break the rule of keeping analog and digital grounds
separate, but in general, there is also a requirement to keep the
voltage difference between digital and analog grounds on a con-
verter as small as possible (typically <0.3 V). The increased
noise, caused by the converter’s digital return currents flowing
through the analog ground plane, will typically be negligible.
Maximum isolation between analog and digital is achieved by
connecting the ground planes back at the supplies. Note that
Figure 31, as drawn, suggests a “star” ground system for the
analog circuitry, with all ground lines being connected, in this
case, to the ADC’s analog ground. However, when ground planes
are used, it is sufficient to connect ground pins to the nearest
point on the low impedance ground plane.
I
SHUNT
R
REF
Figure 30. Operation with a Single Supply
SHUNT
is set to 5 V, the permissible common-mode range
V
REF
REF(–)
V
CM
–V
–IN
+IN
S
( )
±
1
2
3
4
NC = NO CONNECT
=
21.1k
380k
X
380k
V
and V
20
Y
V
V
X Y
AD629
380k
20k
X
/
X
Y
( )
and V
can range between 1 V and
±
−
8
7
6
5
19
NC
+V
REF(+)
Y
S
in Figure 30. These
V
REF
+V
OUTPUT = V
S
0.1 F
OUT
REV. A
–V
REF