Datasheet

AD627 Data Sheet
Rev. E | Page 16 of 24
Table 6. Recommended Values of Gain Resistors
Desired Gain
1% Standard Table
Value of R
G
Resulting Gain
5 ∞ 5.00
6 200 kΩ 6.00
7
100 kΩ
7.00
8 68.1 kΩ 7.94
9 51.1 kΩ 8.91
10 40.2 kΩ 9.98
15 20 kΩ 15.00
20 13.7 kΩ 19.60
25 10 kΩ 25.00
30 8.06 kΩ 29.81
40 5.76 kΩ 39.72
50 4.53 kΩ 49.15
60 3.65 kΩ 59.79
70
3.09 kΩ
69.72
80 2.67 kΩ 79.91
90 2.37 kΩ 89.39
100 2.1 kΩ 100.24
200 1.05 kΩ 195.48
500 412 Ω 490.44
1000 205 Ω 980.61
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 amplifying bipolar signals, because it provides
a virtual ground voltage.
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.
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 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 (V
DIFF
)
that has a common-mode component, V
CM
. The voltage on the
A1 op amp output is a function of V
DIFF
, V
CM
, the voltage on the
REF pin, and the programmed gain. This voltage is given by
V
A1
= 1.25 (V
CM
+ 0.5 V) − 0.25 V
REF
− V
DIFF
(25 kΩ/R
G
+ 0.625) (3)
The voltage on A1 can also be expressed as a function of the
actual voltages on the –IN and +IN pins (V− and V+) such that
V
A1
= 1.25 ((V−) + 0.5 V) − 0.25 V
REF
− ((V+) − (V−)) 25 kΩ/R
G
(4)
The output of A1 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 V
REF
(while it acts as a positive offset at the output of the AD627)
tends to decrease the voltage on A1. Figure 38 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.
V
REF
(V)
V
IN
(–) (V)
5
–6
4
3
2
1
0
–1
–2
–3
–4
–5
–5 –4 –3 –2 –1
0 1 2 3 4
MAXIMUM V
REF
MINIMUM V
REF
00782-036
Figure 38. Reference Input Voltage vs. Negative Input Voltage,
V
S
= ±5 V, G = +5
V
REF
(V)
V
IN
(–) (V)
5
–0.5
4
3
2
1
0
0 0.5 1.0 2.01.5 2.5
3.0 3.5 4.0 4.5
MAXIMUM V
REF
MINIMUM V
REF
00782-037
Figure 39. Reference Input Voltage vs. Negative Input Voltage,
V
S
= 5 V, G = +5
Raising the input common-mode voltage increases 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
because both inputs are shifted upwards by about 0.5 V (that is,
by the V
BE
of Q1 and Q2). Use Equation 3 and Equation 4 to
check whether the voltage on Amplifier A1 is within its
operating range.