Datasheet AD620 (Analog Devices) - 16

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AD620
signal according to the following relationship:
+VS – INPUT
1 FilterFreq = DIFF
AD648
2 R π 2 ( C + C )
100
Ω D C 1
AD620 V
FilterFreq =
R OUT G
CM 2 RC π C
100
Ω
–VS REFERENCE
where CD ≥10CC. 046
+ INPUT
0- CD affects the difference signal. CC affects the common-mode
–VS
00775- signal. Any mismatch in R × CC degrades the AD620 CMRR. To Figure 43. Differential Shield Driver avoid inadvertently reducing CMRR-bandwidth performance, make sure that CC is at least one magnitude smaller than CD. The effect of mismatched CCs is reduced with a larger CD:CC
+VS
ratio.
– INPUT +15V RG 100
Ω
2 0.1
μ
F 1
μ
0 F AD620 V AD548 OUT RG 2 REFERENCE R CC +IN + + INPUT
047 0-
VOUT –V C S D 499
Ω
AD620
00775-
R – –IN REF
Figure 44. Common-Mode Shield Driver
CC GROUNDING 0.1
μ
F 1
μ
0 F
Since the AD620 output voltage is developed with respect to the
–15V
potential on the reference terminal, it can solve many 00775-0-045 grounding problems by simply tying the REF pin to the Figure 42. Circuit to Attenuate RF Interference appropriate “local ground.”
COMMON-MODE REJECTION
To isolate low level analog signals from a noisy digital Instrumentation amplifiers, such as the AD620, offer high environment, many data-acquisition components have separate CMR, which is a measure of the change in output voltage when analog and digital ground pins (Figure 45). It would be both inputs are changed by equal amounts. These specifications convenient to use a single ground line; however, current are usually given for a full-range input voltage change and a through ground wires and PC runs of the circuit card can cause specified source imbalance. hundreds of millivolts of error. Therefore, separate ground For optimal CMR, the reference terminal should be tied to a returns should be provided to minimize the current flow from low impedance point, and differences in capacitance and the sensitive points to the system ground. These ground returns resistance should be kept to a minimum between the two must be tied together at some point, usually best at the ADC inputs. In many applications, shielded cables are used to package shown in Figure 45. minimize noise; for best CMR over frequency, the shield should be properly driven. Figure 43 and Figure 44 show active
ANALOG P.S. DIGITAL P.S.
data guards that are configured to improve ac common-mode
+15V C –15V C +5V
rejections by “bootstrapping” the capacitances of input cable shields, thus minimizing the capacitance mismatch between the
0.1
μ
F 0.1
μ
F 1
μ
F 1
μ
F 1
μ
F
inputs.
+ AD620 DIGITAL AD585 AD574A DATA S/H ADC OUTPUT
048 0- 00775- Figure 45. Basic Grounding Practice Rev. H | Page 16 of 20 Document Outline SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION Make vs. Buy: a Typical Bridge Application Error Budget Pressure Measurement Medical ECG Precision V-I Converter GAIN SELECTION INPUT AND OUTPUT OFFSET VOLTAGE REFERENCE TERMINAL INPUT PROTECTION RF INTERFERENCE COMMON-MODE REJECTION GROUNDING GROUND RETURNS FOR INPUT BIAS CURRENTS AD620ACHIPS INFORMATION OUTLINE DIMENSIONS ORDERING GUIDE