Analog Devices C1599c07 specifications AD620A

AD620

Make vs. Buy: A Typical Bridge Application Error Budget

The AD620 offers improved performance over “homebrew” three op amp IA designs, along with smaller size, fewer compo- nents and 10× lower supply current. In the typical application, shown in Figure 34, a gain of 100 is required to amplify a bridge output of 20 mV full scale over the industrial temperature range of –40°C to +85°C. The error budget table below shows how to calculate the effect various error sources have on circuit accuracy.

Regardless of the system in which it is being used, the AD620 provides greater accuracy, and at low power and price. In simple

systems, absolute accuracy and drift errors are by far the most significant contributors to error. In more complex systems with an intelligent processor, an autogain/autozero cycle will remove all absolute accuracy and drift errors leaving only the resolution errors of gain nonlinearity and noise, thus allowing full 14-bit accuracy.

Note that for the homebrew circuit, the OP07 specifications for input voltage offset and noise have been multiplied by √2. This is because a three op amp type in-amp has two op amps at its inputs, both contributing to the overall input error.

	+10V
				10kV*	10kV*
				OP07D
R = 350V	R = 350V
				10kV**
	RG	AD620A
	499V		100V**	10kV**	OP07D
R = 350V	R = 350V
		REFERENCE
				OP07D
				10kV*	10kV*
PRECISION BRIDGE TRANSDUCER		AD620A MONOLITHIC	“HOMEBREW” IN-AMP, G = 100
		INSTRUMENTATION
		AMPLIFIER, G = 100	*0.02% RESISTOR MATCH, 3PPM/8C TRACKING
			**DISCRETE 1% RESISTOR, 100PPM/8C TRACKING
				SUPPLY CURRENT = 15mA MAX
	SUPPLY CURRENT = 1.3mA MAX

Figure 34. Make vs. Buy

Table I. Make vs. Buy Error Budget

	AD620 Circuit	“Homebrew” Circuit	Error, ppm of Full Scale
Error Source	Calculation	Calculation	AD620	Homebrew
ABSOLUTE ACCURACY at TA = +25°C
Input Offset Voltage, μV	125 μV/20 mV	(150 μV × √2)/20 mV	6,250	10,607
Output Offset Voltage, μV	1000 μV/100/20 mV	((150 μV × 2)/100)/20 mV	500	150
Input Offset Current, nA	2 nA × 350 Ω/20 mV	(6 nA × 350 Ω)/20 mV	18	53
CMR, dB	110 dB→3.16 ppm, × 5 V/20 mV	(0.02% Match × 5 V)/20 mV/100	791	500
		Total Absolute Error	7,558	11,310
DRIFT TO +85°C
Gain Drift, ppm/°C	(50 ppm + 10 ppm) × 60°C	100 ppm/°C Track × 60°C	3,600	6,000
Input Offset Voltage Drift, μV/°C	1 μV/°C × 60°C/20 mV	(2.5 μV/°C × √2 × 60°C)/20 mV	3,000	10,607
Output Offset Voltage Drift, μV/°C	15 μV/°C × 60°C/100/20 mV	(2.5 μV/°C × 2 × 60°C)/100/20 mV	450	150
		Total Drift Error	7,050	16,757
RESOLUTION
Gain Nonlinearity, ppm of Full Scale	40 ppm	40 ppm	40	40
Typ 0.1 Hz–10 Hz Voltage Noise, μV p-p	0.28 μV p-p/20 mV	(0.38 μV p-p × √2)/20 mV	14	27
		Total Resolution Error	54	67
		Grand Total Error	14,662	28,134
G = 100, VS = ± 15 V.
(All errors are min/max and referred to input.)

REV. E –11–