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AD625SD Datasheet(PDF) 9 Page - Analog Devices |
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AD625SD Datasheet(HTML) 9 Page - Analog Devices |
9 / 15 page AD625 REV. D –9– Any resistors in series with the inputs of the AD625 will degrade the noise performance. For this reason the circuit in Figure 26b should be used if the gains are all greater than 5. For gains less than 5, either the circuit in Figure 26a or in Figure 26c can be used. The two 1.4 k Ω resistors in Figure 26a will degrade the noise performance to: 4 kTRext +(4 nV/ Hz)2 = 7.9 nV/ Hz RESISTOR PROGRAMMABLE GAIN AMPLIFIER In the resistor-programmed mode (Figure 27), only three exter- nal resistors are needed to select any gain from 1 to 10,000. Depending on the application, discrete components or a pretrimmed network can be used. The gain accuracy and gain TC are primarily determined by the external resistors since the AD625C contributes less than 0.02% to gain error and under 5 ppm/ °C gain TC. The gain sense current is insensitive to common-mode voltage, making the CMRR of the resistor pro- grammed AD625 independent of the match of the two feedback resistors, RF. Selecting Resistor Values As previously stated each RF provides feedback to the input stage and sets the unity gain transconductance. These feedback resistors are provided by the user. The AD625 is tested and specified with a value of 20 k Ω for RF. Since the magnitude of RTO errors increases with increasing feedback resistance, values much above 20 k Ω are not recommended (values below 10 kΩ for RF may lead to instability). Refer to the graph of RTO noise, offset, drift, and bandwidth (Figure 28) when selecting the feedback resistors. The gain resistor (RG) is determined by the formula RG = 2 RF/(G – l). +GAIN SENSE –GAIN SENSE +INPUT –INPUT RTI NULL RTI NULL RTO NULL RTO NULL +VS +GAIN DRIVE –GAIN DRIVE RF RG RF NC REF –VS VOUT +VS G = +1 2RF RG A1 A2 AD625 10k 10k 10k 10k A3 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 Figure 27. AD625 in Fixed Gain Configuration A list of standard resistors which can be used to set some com- mon gains is shown in Table I. For single gain applications, only one offset null adjust is neces- sary; in these cases the RTI null should be used. RTO OFFSET VOLTAGE DRIFT 6 5 4 3 2 1 60k 50k 40k 30k 20k 10k BANDWIDTH 1M 100k 10k 1 10 100 1k 10k 20k 50k FEEDBACK RESISTANCE – FEEDBACK RESISTANCE – RTO NOISE RTO OFFSET VOLTAGE 300 200 100 3 2 10k 20k 30k 40k 50k 60k 10k 20k 30k 40k 50k 60k FEEDBACK RESISTANCE – FEEDBACK RESISTANCE – Figure 28. RTO Noise, Offset, Drift and Bandwidth vs. Feedback Resistance Normalized to 20 k Ω Table I. Common Gains Nominally Within 0.5% Error Using Standard 1% Resistors GAIN RF RG 1 20 k Ω∞ 2 19.6 k Ω 39.2 k Ω 5 20 k Ω 10 k Ω 10 20 k Ω 4.42 k Ω 20 20 k Ω 2.1 k Ω 50 19.6 k Ω 806 Ω 100 20 k Ω 402 Ω 200 20.5 k Ω 205 Ω 500 19.6 k Ω 78.7 Ω 1000 19.6 k Ω 39.2 Ω 4 20 k Ω 13.3 k Ω 8 19.6 k Ω 5.62 k Ω 16 20 k Ω 2.67 k Ω 32 19.6 k Ω 1.27 k Ω 64 20 k Ω 634 Ω 128 20 k Ω 316 Ω 256 19.6 k Ω 154 Ω 512 19.6 k Ω 76.8 Ω 1024 19.6 k Ω 38.3 Ω SENSE TERMINAL The sense terminal is the feedback point for the AD625 output amplifier. Normally it is connected directly to the output. If heavy load currents are to be drawn through long leads, voltage drops through lead resistance can cause errors. In these in- stances the sense terminal can be wired to the load thus putting |
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