Datasheet

Data Sheet AD627
Rev. E | Page 15 of 24
USING THE AD627
BASIC CONNECTIONS
Figure 36 shows the basic connection circuit for the AD627.
The +V
S
and −V
S
terminals connect to the power supply. The
supply can be either bipolar (V
S
= ±1.1 V to ±18 V) or single
supply (−V
S
= 0 V, +V
S
= 2.2 V to 36 V). Capacitively decouple
the power supplies close to the power pins of the device. For
best results, use surface-mount 0.1 µF ceramic chip capacitors.
The input voltage can be single-ended (tie either −IN or +IN to
ground) or differential. The difference between the voltage on the
inverting and noninverting pins is amplified by the programmed
gain. The gain resistor programs the gain as described in the
Setting the Gain and Reference Terminal sections. Basic
connections are shown in Figure 36. The output signal appears
as the voltage difference between the output pin and the
externally applied voltage on the REF pin, as shown in Figure 37.
SETTING THE GAIN
The gain of the AD627 is resistor programmed by R
G
, or, more
precisely, by whatever impedance appears between Pin 1 and Pin 8.
The gain is set according to
Gain = 5 + (200 kΩ/R
G
) or R
G
= 200 kΩ/(Gain − 5) (2)
Therefore, the minimum achievable gain is 5 (for 200 kΩ/
(Gain − 5)). With an internal gain accuracy of between 0.05%
and 0.7%, depending on gain and grade, a 0.1% external gain
resistor is appropriate to prevent significant degradation of the
overall gain error. However, 0.1% resistors are not available in a
wide range of values and are quite expensive. Table 6 shows
recommended gain resistor values using 1% resistors. For all
gains, the size of the gain resistor is conservatively chosen as the
closest value from the standard resistor table that is higher than
the ideal value. This results in a gain that is always slightly less
than the desired gain, thereby preventing clipping of the signal
at the output due to resistor tolerance.
The internal resistors on the AD627 have a negative temperature
coefficient of −75 ppm/°C maximum for gains > 5. Using a
gain resistor that also has a negative temperature coefficient
of −75 ppm/°C or less tends to reduce the overall gain drift of
the circuit.
+IN
V
OUT
V
IN
–IN
REF (INPUT)
–1.1V TO –18V
+1.1V TO +18V
R
G
+V
S
–V
S
R
G
R
G
OUTPUT
REF
0.1µF
0.1µF
+IN
V
OUT
V
IN
–IN
REF (INPUT)
GAIN = 5 + (200kΩ/R
G
)
+2.2V TO +36V
R
G
+V
S
R
G
R
G
OUTPUT
REF
0.1µF
00782-034
Figure 36. Basic Connections for Single and Dual Supplies
R
G
EXTERNAL GAIN RESISTOR
REF
–IN
+IN
–V
S
–V
S
+V
S
+V
S
100kΩ
25kΩ 25kΩ
200kΩ
200kΩ
2kΩ
2kΩ
A1
0.1V V
A
A2
OUTPUT
–V
S
100kΩ
–IN
+IN
V+
V–
V
DIFF
2
V
DIFF
2
V
CM
Q1 Q2
00782-035
Figure 37. Amplifying Differential Signals with a Common-Mode Component