Datasheet AD621 (Analog Devices) - 14

AD621 Gain Selection +VS +VS
The AD621 has accurate, low temperature coefficient (TC), gains of 10 and 100 available. The gain of the AD621 is nomi-
0.1
␮
F 0.1
␮
F –
nally set at 10; this is easily changed to a gain of 100 by simply
–
connecting a jumper between Pins 1 and 8.
INPUTS AD621 – + OUTPUT + AD526 + 2 G = 10 555.5
⍀
20k
⍀
0.1
␮
F R 5,555.5 –V EXT
⍀
AD621 S 0.1
␮
F –VS
Figure 12. A High Performance Programmable Gain Amplifier Figure 11. Programming the AD621 for Gains Between
COMMON-MODE REJECTION
10 and 100 Instrumentation amplifiers like the AD621 offer high CMR As shown in Figure 11, the device can be programmed for any which is a measure of the change in output voltage when both gain between 10 and 100 by connecting a single external resistor inputs arc changed by equal amounts. These specifications are between Pins 1 and 8. Note that adding the external resistor will usually given for a full-range input voltage change and a speci- degrade both the gain accuracy and gain TC. Since the gain fied source imbalance. equation of the AD621 yields: For optimal CMR, the reference terminal should be tied to a 9 ( R + 6,111.111) low impedance point, and differences in capacitance and resis- G = 1 + X tance should be kept to a minimum between the two inputs. In ( R + 555.555) X many applications shielded cables are used to minimize noise, This can be solved for the nominal value of external resistor for and for best CMR over frequency the shield should he properly gains between 10 and 100: driven. Figures 13 and 14 show active data guards that are config- ured to improve ac common-mode rejections by “bootstrapping” R = (G – 1) 555.555 – 55,000 the capacitances of input cable shields, thus minimizing the X (10 – G ) capacitance mismatch between the inputs. Table III gives practical 1% resistor values for several com-
+VS
mon gains.
–INPUT – 100
⍀
AD648 Table III. Practical 1% External Resistor 100k
⍀
Values for Gains Between 10 and 100 VOUT 100k
⍀
AD621 Desired Recommended Temperature 100
⍀
–V Gain 1% Resistor Value Gain Error Coefficient (TC) S REFERENCE
10 ∞ (Pins 1 and 8 Open) * 5 ppm/°C max
+ +INPUT
20 4.42 kΩ ±10% ≈0.4 (50 ppm/°C
–VS
+ Resistor TC) 50 698 Ω ±10% ≈0.4 (50 ppm/°C Figure 13. Differential Shield Driver, G = 10 + Resistor TC) 100 0 (Pins 1 and 8 Shorted) * 5 ppm/°C max
+VS
*Factory trimmed–exact value depends on grade.
– INPUT 2 A High Performance Programmable Gain Amplifier 7 1
The excellent performance of the AD621 at a gain of 10 makes
VOUT 100
⍀ it a good choice to team up with the AD526 programmable gain
AD548 AD621 6
amplifier (PGA) to yield a differential input PGA with gains of
5 8
10, 20, 40, 80, 160. As shown in Figure 12, the low offset of the
REFERENCE 4 3
AD621 allows total circuit offset to be trimmed using the offset
+ INPUT
null of the AD526, with only a negligible increase in total drift
–VS
error. The total gain TC will be 9 ppm/°C max, with 2 µV/°C Figure 14. Common-Mode Shield Driver, G = 100 typical input offset drift. Bandwidth is 600 kHz to gains of 10 to 80, and 350 kHz at G = 160. Settling time is 13 µs to 0.01% for a 10 V output step for all gains. –14– REV. B