AD627A AD [Analog Devices], AD627A Datasheet - Page 11

• Manufacturer Part Number: AD627A
• Description: Micropower, Single and Dual Supply Rail-to-Rail Instrumentation Amplifier
• Manufacturer: AD [Analog Devices]

V
V– = 0 V, V+ = 0 V to 1 V
V– = 0 V, V+ = 0 mV to 100 mV
V– = 0 V, V+ = 0 mV to 10 mV
Desired
Gain
5
6
7
8
9
10
15
20
25
30
40
50
60
70
80
90
100
200
500
1000
Reference Terminal
The reference terminal potential defines the zero output voltage
and is especially useful when the load does not share a precise
ground with the rest of the system. It provides a direct means of
injecting a precise offset to the output. The reference terminal is
also useful when bipolar signals are being amplified as it can be
used to provide a virtual ground voltage.
Since the AD627 output voltage is developed with respect to the
potential on the reference terminal, it can solve many grounding
problems by simply tying the REF pin to the appropriate “local
ground.” The REF pin should however be tied to a low imped-
ance point for optimal CMR.
REV. A
100 mV, V
50 mV, V
10 mV, V
IN
Table I. Recommended Values of Gain Resistors
CM
CM
CM
= 0 V
= 0 V
= 0 V
V
CM
1% Std Table
Value of R
200 k
100 k
68.1 k
51.1 k
40.2 k
20 k
13.7 k
10 k
8.06 k
5.76 k
4.53 k
3.65 k
3.09 k
2.67 k
2.37 k
2.1 k
1.05 k
412
205
Table II. Maximum Gain for Low Common-Mode Single Supply Applications
Figure 34. Amplifying Differential Signals with a Common-Mode Component
V
V
DIFF
DIFF
2
2
G
V–
V+
,
+IN
–IN
REF
Pin
2 V
2 V
2 V
1 V
1 V
1 V
REF
–IN
2k
Resulting
Gain
5
6
7
7.93
8.91
9.98
15
19.6
25
29.81
39.72
49.15
59.79
69.73
79.9
89.39
99.24
195.48
489.44
980.61
Supply
Voltage
+5 V to +15 V
+5 V to +15 V
+5 V to +15 V
+10 V to +15 V
+5 V to +15 V
+5 V to +15 V
100k
+V
–V
S
S
Q1
EXTERNAL GAIN RESISTOR
200k
–11–
25k
A1
Input Range Limitations in Single Supply Applications
In general, the maximum achievable gain is determined by the
available output signal range. However, in single supply applica-
tions where the input common mode voltage is close to or equal
to zero, some limitations on the gain can be set. While the In-
put, Output and Reference Pins have ranges that are nominally
defined on the specification pages, there is a mutual interdepen-
dence between the voltage ranges on these pins. Figure 34 shows
the simplified schematic of the AD627, driven by a differential
voltage V
voltage on the output of op amp A1 is a function of V
the voltage on the REF pin and the programmed gain. This
voltage is given by the equation:
We can also express the voltage on A1 as a function of the ac-
tual voltages on the –IN and +IN pins (V– and V+)
A1’s output is capable of swinging to within 50 mV of the nega-
tive rail and to within 200 mV of the positive rail. From either of
the above equations, it is clear that an increasing V
acts as a positive offset at the output of the AD627), tends to
decrease the voltage on A1. Figures 35 and 36 show the maxi-
mum voltages that can be applied to the REF pin, for a gain of
five for both the single and dual supply cases. Raising the input
common-mode voltage will increase the voltage on the output of
A1. However, in single supply applications where the common-
mode voltage is low, a differential input voltage or a voltage on
REF that is too high can drive the output of A1 into the ground
rail. Some low side headroom is added by virtue of both inputs
being shifted upwards by about 0.5 V (i.e., by the V
and Q2). The above equations can be used to check that the
voltage on amplifier A1 is within its operating range.
Table II gives values for the maximum gains for various single
supply input conditions. The resulting output swings shown
refer to 0 V. The voltages on the REF pins has been set to either
V
A1
V
R
A1
= 1.25 (V
G
= 1.25 (V– + 0.5 V) – 0.25 V
R
Tolerance)
28.7 k
10.7 k
1.74 k
78.7 k
7.87 k
7.87
25k
G
DIFF
V
(1%
A
Q2
which has a common mode component, V
CM
200k
+ 0.5 V) – 0.25 V
+V
–V
S
S
100k
2k
A2
Resulting
Max Gain
12.0
23.7
119.9
7.5
31
259.1
+IN
REF
REF
OUTPUT
–V
– V
S
– (V+ – V–) 25 k /R
DIFF
(25 k /R
Output Swing
WRT 0 V
0.8 V to 3.2 V
0.8 V to 3.2 V
0.8 V to 3.2 V
1 V to 8.5 V
1 V to 4.1 V
1 V to 3.6 V
AD627
REF
BE
DIFF
, (while it
G
of Q1
– 0.625)
CM
, V
. The
CM
G
,