AD627B Analog Devices, AD627B Datasheet - Page 10

• Manufacturer Part Number: AD627B
• Description: Micropower/ Single and Dual Supply Rail-to-Rail Instrumentation Amplifier
• Manufacturer: Analog Devices

AD627
THEORY OF OPERATION
The AD627 is a true “instrumentation amplifier” built using
two feedback loops. Its general properties are similar to those of
the classic “two op amp” instrumentation amplifier configura-
tion, and can be regarded as such, but internally the details are
somewhat different. The AD627 uses a modified “current feed-
back” scheme which, coupled with interstage feedforward
frequency compensation, results in a much better CMRR
(Common-Mode Rejection Ratio) at frequencies above dc (no-
tably the line frequency of 50 Hz–60 Hz) than might otherwise
be expected of a low power instrumentation amplifier.
Referring to the diagram, (Figure 32), A1 completes a feedback
loop which, in conjunction with V1 and R5, forces a constant
collector current in Q1. Assume that the gain-setting resistor
(R
plete the loop and force the output of A1 to be equal to the
voltage on the inverting terminal with a gain of (almost exactly)
1.25. A nearly identical feedback loop completed by A2 forces a
current in Q2 which is substantially identical to that in Q1, and
A2 also provides the output voltage. When both loops are bal-
anced, the gain from the noninverting terminal to V
to 5, whereas the gain from the output of A1 to V
–4. The inverting terminal gain of A1, (1.25) times the gain of
A2, (–4) makes the gain from the inverting and noninverting
terminals equal.
The differential mode gain is equal to 1 + R4/R3, nominally five
and is factory trimmed to 0.01% final accuracy. Adding an external
gain setting resistor (R
to (R4 + R1)/R
following equation.
REF
–IN
G
) is not present for the moment. Resistors R2 and R1 com-
V
OUT
2k
100k
R1
= [V
+V
–V
S
S
Figure 32. Simplified Schematic
G
. The output voltage of the AD627 is given by the
IN
Q1
(+) – V
EXTERNAL GAIN RESISTOR
R5
200k
25k
R2
G
A1
V
) increases the gain by an amount equal
IN
IN
+IN
–IN
V1
(–)] (5 + 200 k /R
R
R
G
G
25k
R3
R
R
Figure 33. Basic Connections for Single and Dual Supplies
Q2
G
G
+V
–V
S
S
R6
200k
OUTPUT
REF
0.1 F
0.1 F
+V
–V
–1.1V TO –18V
+1.1V TO +18V
S
S
100k
2k
R4
A2
G
+IN
OUT
) + V
OUT
V
REF (INPUT)
OUT
is equal to
REF
is equal
OUTPUT
–V
GAIN = 5 + (200k /R
S
–10–
Laser trims are performed on R1 through R4 to ensure that
their values are as close as possible to the absolute values in the
gain equation. This ensures low gain error and high common-
mode rejection at all practical gains.
USING THE AD627
Basic Connections
Figure 33 shows the basic connection circuit for the AD627.
The +V
The supply can either be bipolar (V
single supply (–V
supplies should be capacitively decoupled close to the devices
power pins. For best results, use surface mount 0.1 F ceramic
chip capacitors.
The input voltage, which can be either 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 programmed gain is set by the
gain resistor (see below). The output signal appears as the volt-
age difference between the output pin and the externally applied
voltage on the REF pin (see below).
Setting the Gain
The AD627s gain is resistor programmed by R
cisely, by whatever impedance appears between Pins 1 and 8.
The gain is set according to the equation:
or
It follows that the minimum achievable gain is 5 (for R
With an internal gain accuracy of between 0.05% and 0.7%
depending on gain and grade, a 0.1% external gain resistor
would seem 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 I 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. This prevents clipping of the signal at the
output due to resistor tolerance.
The internal resistors on the AD627 have a negative tempera-
ture coefficient of –75 ppm/ C max for gains > 5. Using a gain
resistor that also has a negative temperature coefficient of
–75 ppm/ C or less will tend to reduce the overall circuit’s gain
drift.
V
IN
G
)
+IN
–IN
R
S
G
and –V
R
R
G
G
+V
S
S
OUTPUT
S
REF
0.1 F
terminals are connected to the power supply.
= 0 V, +V
Gain = 5 + (200 k /R
R
+2.2V TO +36V
G
= 200 k /(Gain – 5)
S
= +2.2 V to +36 V). The power
V
REF (INPUT)
OUT
S
= 1.1 V to 18 V) or
G
)
G
, or more pre-
G
REV. A
=
).