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AD8202YR-REEL7 Scheda tecnica(PDF) 10 Page - Analog Devices |
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AD8202YR-REEL7 Scheda tecnica(HTML) 10 Page - Analog Devices |
10 / 12 page AD8202 Rev. A | Page 10 of 12 GAIN TRIM Figure 18 shows a method for incremental gain trimming by using a trim potentiometer and external resistor REXT. The following approximation is useful for small gain ranges. ΔG ≈ (10 MΩ ÷ REXT)% Thus, the adjustment range is ±2% for REXT = 5 MΩ; ±10% for REXT = 1 MΩ, and so on. 5V OUT REXT GAIN TRIM 20k Ω MIN VCM VDIFF 2 VDIFF 2 NC = NO CONNECT GND NC –IN +IN A1 +VS A2 OUT AD8202 Figure 18. Incremental Gain Trim Internal Signal Overload Considerations When configuring gain for values other than 20, the maximum input voltage with respect to the supply voltage and ground must be considered, since either the preamplifier or the output buffer reaches its full-scale output (approximately VS – 0.2 V) with large differential input voltages. The input of the AD8202 is limited to (VS – 0.2) ÷ 10 for overall gains ≤ 10, since the pre- amplifier, with its fixed gain of ×10, reaches its full-scale output before the output buffer. For gains greater than 10, the swing at the buffer output reaches its full scale first and limits the AD8202 input to (VS – 0.2) ÷ G, where G is the overall gain. LOW-PASS FILTERING In many transducer applications, it is necessary to filter the sig- nal to remove spurious high frequency components including noise, or to extract the mean value of a fluctuating signal with a peak-to-average ratio (PAR) greater than unity. For example, a full-wave rectified sinusoid has a PAR of 1.57, a raised cosine has a PAR of 2, and a half-wave sinusoid has a PAR of 3.14. Signals having large spikes may have PARs of 10 or more. When implementing a filter, the PAR should be considered so that the output of the AD8202 preamplifier (A1) does not clip before A2, since this nonlinearity would be averaged and appear as an error at the output. To avoid this error, both amplifiers should be made to clip at the same time. This condition is achieved when the PAR is no greater than the gain of the sec- ond amplifier (2 for the default configuration). For example, if a PAR of 5 is expected, the gain of A2 should be increased to 5. Low-pass filters can be implemented in several ways by using the features provided by the AD8202. In the simplest case, a single-pole filter (20 dB/decade) is formed when the output of A1 is connected to the input of A2 via the internal 100 kΩ resis- tor by strapping Pins 3 and 4 and a capacitor added from this node to ground, as shown in Figure 19. If a resistor is added across the capacitor to lower the gain, the corner frequency increases; it should be calculated using the parallel sum of the resistor and 100 kΩ. 5V VCM VDIFF 2 VDIFF 2 NC = NO CONNECT C GND NC –IN +IN A1 +VS A2 OUT AD8202 OUTPUT FC = 1 2 πC105 C IN FARADS Figure 19. Single-Pole, Low-Pass Filter Using the Internal 100 kΩ Signal If the gain is raised using a resistor, as shown in Figure 17, the corner frequency is lowered by the same factor as the gain is raised. Thus, using a resistor of 200 kΩ (for which the gain would be doubled), the corner frequency is now 0.796 Hz µF (0.039 µF for a 20 Hz corner frequency.) 5V VCM VDIFF 2 VDIFF 2 NC = NO CONNECT C GND NC –IN +IN A1 +VS A2 OUT AD8202 OUT C 255k Ω FC = 1Hz – µF Figure 20. 2-Pole, Low-Pass Filter A 2-pole filter (with a roll-off of 40 dB/decade) can be imple- mented using the connections shown in Figure 20. This is a Sallen-Key form based on a ×2 amplifier. It is useful to remember that a 2-pole filter with a corner frequency f2 and a 1-pole filter with a corner at f1 have the same attenuation at the frequency (f22/f1). The attenuation at that frequency is 40 log (f2/f1), which is illustrated in Figure 21. Using the standard resistor value shown and equal capacitors (Figure 20), the corner frequency is conven- iently scaled at 1 Hz µF (0.05 µF for a 20 Hz corner). A maximally flat response occurs when the resistor is lowered to 196 kΩ and the scaling is then 1.145 Hz µF. The output offset is raised by approximately 5 mV (equivalent to 250 µV at the input pins). |
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