Analog Devices C1599c07, AD620 specifications COMMON-MODE Rejection, Grounding

AD620

COMMON-MODE REJECTION

Instrumentation amplifiers like the AD620 offer high CMR, which is a measure of the change in output voltage when both inputs are changed by equal amounts. These specifications are usually given for a full-range input voltage change and a speci- fied source imbalance.

For optimal CMR the reference terminal should be tied to a low impedance point, and differences in capacitance and resistance should be kept to a minimum between the two inputs. In many applications shielded cables are used to minimize noise, and for best CMR over frequency the shield should be properly driven. Figures 39 and 40 show active data guards that are configured to improve ac common-mode rejections by “bootstrapping” the capacitances of input cable shields, thus minimizing the capaci- tance mismatch between the inputs.

		+VS
	– INPUT
100V	AD648
	RG	AD620	VOUT
100V	–V
	S
			REFERENCE

+ INPUT

–V

S

Figure 39. Differential Shield Driver

GROUNDING

Since the AD620 output voltage is developed with respect to the potential on the reference terminal, it can solve many grounding problems by simply tying the REF pin to the appropriate “local ground.”

In order to isolate low level analog signals from a noisy digital environment, many data-acquisition components have separate analog and digital ground pins (Figure 41). It would be conve- nient to use a single ground line; however, current through ground wires and PC runs of the circuit card can cause hun- dreds of millivolts of error. Therefore, separate ground returns should be provided to minimize the current flow from the sensi- tive points to the system ground. These ground returns must be tied together at some point, usually best at the ADC package as shown.

ANALOG P.S.	DIGITAL P.S.
+15V C –15V	C +5V

0.1mF	0.1mF
	1mF	1mF	1mF
			+
AD620	AD585	AD574A	DIGITAL
			DATA
	S/H	ADC
			OUTPUT

Figure 41. Basic Grounding Practice

	– INPUT
		RG
100V	AD548	2
		RG
		2
	+ INPUT

+VS

AD620	VOUT
	REFERENCE
–V
S

Figure 40. Common-Mode Shield Driver

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