Analog Devices C1599c07 specifications AD620, Theory of Operation

AD620

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 31b. Gain Nonlinearity, G = 100, RL = 10 kΩ

(100 μV = 10 ppm)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 31c. Gain Nonlinearity, G = 1000, RL = 10 kΩ
(1 mV = 100 ppm)
			10kV*	1kV	10kV
INPUT				10T
10V p-p
100kV					VOUT
				+VS
11kV	1kV	100V	2
			1	7
	G=1000		G=1
		G=100	G=10	AD620	6
	49.9V 499V		5.49kV		5
			8
				4
			3

I1	20mA	VB	20mA	I2
	A1		A2		10kV
	C1		C2
				10kV
					A3	OUTPUT
R3	R1		R2	10kV	10kV	REF
400V
– IN	Q1			Q2	+IN
		RG		R4
				400V
	GAIN		GAIN
	SENSE		SENSE
		–V
		S

Figure 33. Simplified Schematic of AD620

THEORY OF OPERATION

The AD620 is a monolithic instrumentation amplifier based on a modification of the classic three op amp approach. Absolute value trimming allows the user to program gain accurately (to 0.15% at G = 100) with only one resistor. Monolithic construc- tion and laser wafer trimming allow the tight matching and tracking of circuit components, thus ensuring the high level of performance inherent in this circuit.

The input transistors Q1 and Q2 provide a single differential- pair bipolar input for high precision (Figure 33), yet offer 10× lower Input Bias Current thanks to Superβeta processing. Feed- back through the Q1-A1-R1 loop and the Q2-A2-R2 loop main- tains constant collector current of the input devices Q1, Q2 thereby impressing the input voltage across the external gain setting resistor RG. This creates a differential gain from the inputs to the A1/A2 outputs given by G = (R1 + R2)/RG + 1. The unity-gain subtracter A3 removes any common-mode sig- nal, yielding a single-ended output referred to the REF pin potential.

The value of RG also determines the transconductance of the preamp stage. As RG is reduced for larger gains, the transcon- ductance increases asymptotically to that of the input transistors. This has three important advantages: (a) Open-loop gain is boosted for increasing programmed gain, thus reducing gain- related errors. (b) The gain-bandwidth product (determined by C1, C2 and the preamp transconductance) increases with pro- grammed gain, thus optimizing frequency response. (c) The input voltage noise is reduced to a value of 9 nV/√Hz, deter- mined mainly by the collector current and base resistance of the input devices.

The internal gain resistors, R1 and R2, are trimmed to an abso- lute value of 24.7 kΩ, allowing the gain to be programmed accurately with a single external resistor.

The gain equation is then

–V

S

*ALL RESISTORS 1% TOLERANCE

Figure 32. Settling Time Test Circuit

= 8>28 Ω +

so that

5

=8>28 Ω

−5

–10– REV. E