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AD8200YR-REEL Folha de dados(PDF) 6 Page - Analog Devices |
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AD8200YR-REEL Folha de dados(HTML) 6 Page - Analog Devices |
6 / 12 page REV. B –6– AD8200 CURRENT SENSING High Line, High Current Sensing Basic automotive applications making use of the large common- mode range are shown in Figures 1 and 2. The capability of the device to operate as an amplifier in primary battery supply circuits is shown in Figure 1; Figure 2 illustrates the ability of the device to withstand voltages below system ground. Low Current Sensing The AD8200 can also be used in low current sensing applica- tions, such as the 4–20 mA current loop shown in Figure 4. In such applications, the relatively large shunt resistor can degrade the common-mode rejection. Adding a resistor of equal value in the low impedance side of the input corrects for this error. 5V OUTPUT 10 10 1% NC = NO CONNECT 1% + GND NC –IN +IN A1 +VS A2 OUT AD8200 Figure 4. 4–20 mA Current Loop Receiver GAIN ADJUSTMENT The default gain of the preamplifier and buffer are 10 and 2, respectively, resulting in a composite gain of 20. With the addition of external resistor(s) or trimmer(s), the gain may be lowered, raised, or finely calibrated. Gains Less than 20 Since the preamplifier has an output resistance of 100 k Ω, an exter- nal resistor connected from Pins 3 and 4 to GND will decrease the gain by a factor REXT/(100 k Ω + R EXT) (see Figure 5). 10k 10k 100k A2 A1 GND –IN OUT +VS NC +IN AD8200 OUT +VS REXT VCM VDIFF 2 GAIN = 20REXT REXT + 100k REXT = 100k GAIN 20 – GAIN VDIFF 2 NC = NO CONNECT Figure 5. Adjusting for Gains Less than 20 The overall bandwidth is unaffected by changes in gain using this method, although there may be a small offset voltage due to the imbalance in source resistances at the input to the buffer. In many cases this can be ignored, but if desired, can be nulled by inserting a resistor equal to 100 k Ω minus the parallel sum of REXT and 100 k Ω, in series with Pin 4. For example, with R EXT = 100 k Ω (yielding a composite gain of 10), the optional offset nulling resistor is 50 k Ω (see Figure 11.) Gains Greater than 20 Connecting a resistor from the output of the buffer amplifier to its noninverting input, as shown in Figure 6, will increase the gain. The gain is now multiplied by the factor REXT/(REXT – 100 k Ω); for example, it is doubled for R EXT = 200 k Ω. Overall gains as high as 50 are achievable in this way. Note that the accuracy of the gain becomes critically dependent on resistor value at high gains. Also, the effective input offset voltage at Pins 1 and 8 (about six times the actual offset of A1) limits the part’s use in very high gain, dc-coupled applications. 10k 10k 100k A2 A1 –IN +VS NC +IN AD8200 GND OUT OUT +VS VCM GAIN = 20REXT REXT – 100k REXT = 100k GAIN GAIN – 20 REXT VDIFF 2 VDIFF 2 NC = NO CONNECT Figure 6. Adjusting for Gains Greater than 20 GAIN TRIM Figure 7 shows a method for incremental gain trimming using a trimpot and external resistor REXT. The following approximation is useful for small gain ranges ∆Ω GM R EXT ≈÷ () 10 % Thus, the adjustment range would be ±2% for R EXT = 5 M Ω; ±10% for REXT = 1 MΩ, and so on. 5V OUT REXT GAIN TRIM 20k MIN VCM VDIFF 2 VDIFF 2 NC = NO CONNECT GND NC –IN +IN A1 +VS A2 OUT AD8200 Figure 7. Incremental Gain Trim |
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