AD624
directly proportional to gain i.e., input offset as measured at
the output at G = 100 is 100 times greater than at G = 1.
Output offset is independent of gain. At low gains, output offset
drift is dominant, while at high gains input offset drift domi-
nates. Therefore, the output offset voltage drift is normally
specified as drift at G = 1 (where input effects are insignificant),
while input offset voltage drift is given by drift specification at a
high gain (where output offset effects are negligible). All input-
related numbers are referred to the input (RTI) which is to say
that the effect on the output is “G” times larger. Voltage offset
vs. power supply is also specified at one or more gain settings
and is also RTI.
By separating these errors, one can evaluate the total error inde-
pendent of the gain setting used. In a given gain configura-
tion both errors can be combined to give a total error referred to
the input (R.T.I.) or output (R.T.O.) by the following formula:
Total Error R.T.I. = input error + (output error/gain)
Total Error R.T.O. = (Gain
×
input error) + output error
As an illustration, a typical AD624 might have a +250
µV
out-
put offset and a –50
µV
input offset. In a unity gain configura-
tion, the
total
output offset would be 200
µV
or the sum of the
two. At a gain of 100, the output offset would be –4.75 mV
or: +250
µV
+ 100 (–50
µV)
= –4.75 mV.
The AD624 provides for both input and output offset adjust-
ment. This optimizes nulling in very high precision applications
and minimizes offset voltage effects in switched gain applica-
tions. In such applications the input offset is adjusted first at the
highest programmed gain, then the output offset is adjusted at
G = 1.
GAIN
Table I.
Gain
(Nominal)
1
100
125
137
186.5
200
250
333
375
500
624
688
831
1000
Temperature
Coefficient
(Nominal)
–0 ppm/°C
–1.5 ppm/°C
–5 ppm/°C
–5.5 ppm/°C
–6.5 ppm/°C
–3.5 ppm/°C
–5.5 ppm/°C
–15 ppm/°C
–0.5 ppm/°C
–10 ppm/°C
–5 ppm/°C
–1.5 ppm/°C
+4 ppm/°C
0 ppm/°C
Pin 3
to Pin
–
13
13
13
13
12
12
12
12
11
11
11
11
11
Connect Pins
–
–
11 to 16
11 to 12
11 to 12 to 16
–
11 to 13
11 to 16
13 to 16
–
13 to 16
11 to 12; 13 to 16
16 to 12
16 to 12; 13 to 11
Pins 3 and 16 programs the gain according to the formula
40k
R
G
=
G
−
1
(see Figure 29). For best results R
G
should be a precision resis-
tor with a low temperature coefficient. An external R
G
affects both
gain accuracy and gain drift due to the mismatch between it and
the internal thin-film resistors R56 and R57. Gain accuracy is
determined by the tolerance of the external R
G
and the absolute
accuracy of the internal resistors (±20%). Gain drift is determined
by the mismatch of the temperature coefficient of R
G
and the tem-
perature coefficient of the internal resistors (–15 ppm/°C typ),
and the temperature coefficient of the internal interconnections.
+V
S
–INPUT
RG
1
1.5k
1k
OR
2.105k
RG
2
+INPUT
–V
S
The AD624 includes high accuracy pretrimmed internal
gain resistors. These allow for single connection program-
ming of gains of 1, 100, 200 and 500. Additionally, a variety
of gains including a pretrimmed gain of 1000 can be achieved
through series and parallel combinations of the internal resis-
tors. Table I shows the available gains and the appropriate
pin connections and gain temperature coefficients.
The gain values achieved via the combination of internal
resistors are extremely useful. The temperature coefficient of the
gain is dependent primarily on the mismatch of the temperature
coefficients of the various internal resistors. Tracking of these
resistors is extremely tight resulting in the low gain TCs shown
in Table I.
If the desired value of gain is not attainable using the inter-
nal resistors, a single external resistor can be used to achieve
any gain between 1 and 10,000. This resistor connected between
+V
S
–INPUT
RG
1
G = 100
G = 200
G = 500
RG
2
+INPUT
–V
S
10k
INPUT
OFFSET
NULL
AD624
V
OUT
REFERENCE
G = 40.000 + 1 = 20
2.105
20%
Figure 29. Operating Connections for G = 20
The AD624 may also be configured to provide gain in the out-
put stage. Figure 30 shows an H pad attenuator connected to
the reference and sense lines of the AD624. The values of R1,
R2 and R3 should be selected to be as low as possible to mini-
mize the gain variation and reduction of CMRR. Varying R2
will precisely set the gain without affecting CMRR. CMRR is
determined by the match of R1 and R3.
+V
S
–INPUT
RG
1
G = 100
G = 200
G = 500
RG
2
+INPUT
–V
S
G=
(R
2
||20k ) + R
1
+ R
3
)
(R
2
||20k )
(R
1
+ R
2
+ R
3
) || R
L
2k
R3
6k
R1
6k
R2
5k
AD624
OUTPUT
SIGNAL
COMMON
V
OUT
AD624
R
L
V
OUT
Figure 28. Operating Connections for G = 200
Figure 30. Gain of 2500
–8–
REV. C
AD [ ANALOG DEVICES ]
