Datasheet

AD629
Rev. C | Page 13 of 16
OUTPUT CURRENT AND BUFFERING
The AD629 is designed to drive loads of 2 kΩ to within 2 V of
the rails but can deliver higher output currents at lower output
voltages (see Figure 17). If higher output current is required, the
output of the AD629 should be buffered with a precision op amp,
such as the OP113, as shown in Figure 38. This op amp can swing
to within 1 V of either rail while driving a load as small as 600 Ω.
REF (–)
REF (+)
–
V
S
–V
S
+V
S
V
OU
T
NC
–IN
+IN
0.1µF
0.1µF
0.1µF
0.1µF
NC = NO CONNECT
21.1kΩ
380kΩ 380kΩ
20kΩ
380kΩ
AD629
1
2
3
4
8
7
6
5
00783-036
OP113
Figure 38. Output Buffering Application
A GAIN OF 19 DIFFERENTIAL AMPLIFIER
While low level signals can be connected directly to the –IN and
+IN inputs of the AD629, differential input signals can also be
connected, as shown in Figure 39, to give a precise gain of 19.
However, large common-mode voltages are no longer permissible.
Cold junction compensation can be implemented using a
temperature sensor, such as the AD590.
REF (–)
REF (+)
+V
S
+V
S
NC
–IN
+IN
0.1µF
NC = NO CONNECT
21.1kΩ
380kΩ 380kΩ
20kΩ
380kΩ
AD629
1
2
3
4
8
7
6
5
00783-037
V
OU
T
V
REF
THERMOCOUPLE
Figure 39. A Gain of 19 Thermocouple Amplifier
ERROR BUDGET ANALYSIS EXAMPLE 1
In the dc application that follows, the 10 A output current from
a device with a high common-mode voltage (such as a power
supply or current-mode amplifier) is sensed across a 1 Ω shunt
resistor (see Figure 40). The common-mode voltage is 200 V,
and the resistor terminals are connected through a long pair of
lead wires located in a high noise environment, for example,
50 Hz/60 Hz, 440 V ac power lines. The calculations in Table 7
assume an induced noise level of 1 V at 60 Hz on the leads, in
addition to a full-scale dc differential voltage of 10 V. The error
budget table quantifies the contribution of each error source.
Note that the dominant error source in this example is due to
the dc common-mode voltage.
REF (–)
OUTPUT
CURRENT
60Hz
POWER LINE
1Ω
SHUNT
REF (+)
–V
S
+V
S
V
OUT
NC
–IN
+IN
0.1µF
0.1µF
NC = NO CONNECT
21.1kΩ
380kΩ 380kΩ
20kΩ
380kΩ
AD629
1
2
3
4
8
7
6
5
00783-038
10 AMPS
200V
CM
DC
TO GROUND
Figure 40. Error Budget Analysis Example 1: V
IN
= 10 V Full-Scale,
V
CM
= 200 V DC, R
SHUNT
= 1 Ω, 1 V p-p, 60 Hz Power-Line Interference
Table 7. AD629 vs. INA117 Error Budget Analysis Example 1 (V
CM
= 200 V dc)
Error, ppm of FS
Error Source AD629 INA117 AD629 INA117
ACCURACY, T
A
= 25°C
Initial Gain Error (0.0005 × 10)/10 V × 10
6
(0.0005 × 10)/10 V × 10
6
500 500
Offset Voltage (0.001 V/10 V) × 10
6
(0.002 V/10 V) × 10
6
100 200
DC CMR (Over Temperature) (224 × 10
-6
× 200 V)/10 V × 10
6
(500 × 10
-6
× 200 V)/10 V × 10
6
4480 10,000
Total Accuracy Error
5080 10,700
TEMPERATURE DRIFT (85°C)
Gain 10 ppm/°C × 60°C 10 ppm/°C × 60°C 600 600
Offset Voltage (20 μV/°C × 60°C) × 10
6
/10 V (40 μV/°C × 60°C) × 10
6
/10 V 120 240
Total Drift Error
720 840
RESOLUTION
Noise, Typical, 0.01 Hz to 10 Hz, μV p-p 15 μV/10 V × 10
6
25 μV/10 V × 10
6
2 3
CMR, 60 Hz (141 × 10
-6
× 1 V)/10 V × 10
6
(500 × 10
-6
× 1 V)/10 V × 10
6
14 50
Nonlinearity (10
-5
× 10 V)/10 V × 10
6
(10
-5
× 10 V)/10 V × 10
6
10 10
Total Resolution Error
26 63
Total Error
5826 11,603