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AD8202YR-REEL7 Datenblatt(PDF) 9 Page - Analog Devices |
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AD8202YR-REEL7 Datenblatt(HTML) 9 Page - Analog Devices |
9 / 12 page AD8202 Rev. A | Page 9 of 12 APPLICATIONS The AD8202 difference amplifier is intended for applications where it is required to extract a small differential signal in the presence of large common-mode voltages. The input resistance is nominally 170 kΩ, and the device can tolerate common-mode voltages higher than the supply voltage and lower than ground. The open collector output stage sources current to within 20 mV of ground and to within 200 mV of VS. CURRENT SENSING High-Line, High Current Sensing Basic automotive applications making use of the large common- mode range are shown in Figure 2 and Figure 3. The capability of the device to operate as an amplifier in primary battery sup- ply circuits is shown in Figure 2; Figure 3 illustrates the ability of the device to withstand voltages below system ground. Low Current Sensing The AD8202 can also be used in low current sensing applications, such as the 4 to 20 mA current loop shown in Figure 15. In such applications, the relatively large shunt resistor can degrade the common-mode rejection. Adding a resistor of equal value on the low impedance side of the input corrects for this error. 5V OUTPUT 10 Ω 1% 10 Ω 1% NC = NO CONNECT + GND NC –IN +IN A1 +VS A2 OUT AD8202 Figure 15. 4 to 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 kΩ, an external resistor connected from Pins 3 and 4 to GND decreases the gain by a factor REXT/(100 kΩ + REXT) (see Figure 16). 10k Ω 10k Ω 100k Ω A2 A1 GND –IN OUT +VS NC +IN AD8202 OUT +VS REXT VCM VDIFF 2 GAIN = 20REXT REXT + 100kΩ REXT = 100kΩ GAIN 20 – GAIN VDIFF 2 NC = NO CONNECT Figure 16. Adjusting for Gains Less than 20 The overall bandwidth is unaffected by changes in gain by 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, it can be nulled by inserting a resistor equal to 100 kΩ minus the parallel sum of REXT and 100 kΩ, in series with Pin 4. For example, with REXT = 100 kΩ (yielding a composite gain of ×10), the optional offset nulling resistor is 50 kΩ. Gains Greater than 20 Connecting a resistor from the output of the buffer amplifier to its noninverting input, as shown in Figure 17, increases the gain. The gain is now multiplied by the factor REXT/(REXT − 100 kΩ); for example, it is doubled for REXT = 200 kΩ. Overall gains as high as 50 are achievable in this way. Note that the accuracy of the gain becomes critically dependent on the resistor value at high gains. Also, the effective input offset voltage at Pin 1 and Pin 8 (about six times the actual offset of A1) limits the part’s use in high gain, dc-coupled applications. 10k Ω 10k Ω 100k Ω A2 A1 GND –IN OUT +VS NC +IN AD8202 OUT +VS REXT VCM VDIFF 2 GAIN = 20REXT REXT – 100kΩ REXT = 100kΩ GAIN GAIN – 20 VDIFF 2 NC = NO CONNECT Figure 17. Adjusting for Gains Greater than 20 |
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