AD627
The voltage on A± can also be expressed as a function of the
actual voltages on the –IN and +IN pins (V− and V+ꢀ such that
Table 6. Recommended Values of Gain Resistors
VA1 = ±.25 ((V−ꢀ + 0.5 V) − 0.25 VREF − ((V+ꢀ − (V−ꢀꢀ 25 kΩ/RG (4ꢀ
1% Standard Table
Desired Gain
Value of RG
Resulting Gain
5.00
6.00
7.00
7.94
The output of A± is capable of swinging to within 50 mV of the
negative rail and to within 200 mV of the positive rail. It is clear,
from either Equation 3 or Equation 4, that an increasing VREF
(while it acts as a positive offset at the output of the AD627ꢀ
tends to decrease the voltage on A±. Figure 31 and Figure 39
show the maximum voltages that can be applied to the REF pin
for a gain of 5 for both the single-supply and dual-supply cases.
5
5
6
7
8
∞
200 kΩ
100 kΩ
68.1 kΩ
51.1 kΩ
40.2 kΩ
20 kΩ
9
8.91
9.98
10
15
20
25
30
40
50
60
70
80
90
100
200
500
1000
15.00
19.60
25.00
29.81
39.72
49.15
59.79
69.72
79.91
89.39
100.24
195.48
490.44
980.61
13.7 kΩ
10 kΩ
4
3
2
8.06 kΩ
5.76 kΩ
4.53 kΩ
3.65 kΩ
3.09 kΩ
2.67 kΩ
2.37 kΩ
2.1 kΩ
MAXIMUM V
REF
1
0
–1
–2
–3
–4
–5
MINIMUM V
REF
1.05 kΩ
412 Ω
205 Ω
0
1
2
3
4
–6
–5
–4
–3
–2
–1
V
(–) (V)
IN
Figure 38. Reference Input ꢀoltage vs. Negative Input ꢀoltage,
ꢀS = ± ± ꢀ, G = +±
REFERENCE TERMINAL
5
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 amplifying bipolar signals, because it provides
a virtual ground voltage.
MAXIMUM V
REF
4
3
2
1
The AD627 output voltage is developed with respect to the poten-
tial on the reference terminal; therefore, tying the REF pin to the
appropriate local ground solves many grounding problems. For
optimal CMR, tie the REF pin to a low impedance point.
MINIMUM V
REF
INPUT RANGE LIMITATIONS IN SINGLE-SUPPLY
APPLICATIONS
0
–0.5
3.0
3.5
4.0
4.5
0
0.5
1.0
1.5
2.0
2.5
V
(–) (V)
IN
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 nearly or equal
to 0, some limitations on the gain can be set. Although the
Specifications section nominally defines the input, output, and
reference pin ranges, the voltage ranges on these pins are
mutually interdependent. Figure 37 shows the simplified
schematic of the AD627, driven by a differential voltage (VDIFFꢀ
that has a common-mode component, VCM. The voltage on the
A± op amp output is a function of VDIFF, VCM, the voltage on the
REF pin, and the programmed gain. This voltage is given by
Figure 39. Reference Input ꢀoltage vs. Negative Input ꢀoltage,
ꢀS = ± ꢀ, G = +±
Raising the input common-mode voltage increases the voltage
on the output of A±. 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 A± into the ground rail. Some low-side headroom is added
because both inputs are shifted upwards by about 0.5 V (that is,
by the VBE of Q± and Q2ꢀ. Use Equation 3 and Equation 4 to
check whether the voltage on Amplifier A± is within its
operating range.
V
A1 = ±.25 (VCM + 0.5 Vꢀ − 0.25 VREF − VDIFF (25 kΩ/RG − 0.625ꢀ (3ꢀ
Rev. D | Page 16 of 24
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