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AD8200 Scheda tecnica(PDF) 7 Page - Analog Devices |
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AD8200 Scheda tecnica(HTML) 7 Page - Analog Devices |
7 / 12 page REV. B AD8200 –7– 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 will reach its full-scale output (approximately VS – 0.2 V) with large differential input voltages. The input of the AD8200 is limited to (VS – 0.2) ÷ 10, for overall gains ≤10, since the preamplifier, 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 AD8200 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 signal 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 the output of the AD8200 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 second ampli- fier (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 using the features provided by the AD8200. 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 k Ω resistor by strapping Pins 3 and 4 and a capacitor added from this node to ground, as shown in Figure 8. If a resistor is added across the capacitor to lower the gain, the corner frequency will increase; it should be calculated using the parallel sum of the resistor and 100 k Ω. 5V C VCM OUT FC = 1 2 C105 C IN FARADS VDIFF 2 VDIFF 2 NC = NO CONNECT GND NC –IN +IN A1 +VS A2 OUT AD8200 Figure 8. A Single-Pole, Low-Pass Filter Using the Internal 100 k Ω Resistor If the gain is raised using a resistor, as shown in Figure 8, the corner frequency is lowered by the same factor as the gain is raised. Thus, using a resistor of 200 k Ω (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.) OUT VCM C 255k C FC = 1Hz – F 5V NC = NO CONNECT VDIFF 2 VDIFF 2 GND NC –IN +IN A1 +VS A2 OUT AD8200 Figure 9. 2-Pole Low-Pass Filter A 2-pole filter (with a roll-off of 40 dB/decade) can be implemented using the connections shown in Figure 9. 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 (f2 2/f 1). The attenuation at that frequency is 40 log (f2/f1). This is illustrated in Figure 10. Using the standard resistor value shown and equal capacitors (Figure 9), the corner frequency is conveniently 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 k Ω and the scaling is then 1.145 Hz- µF. The output offset is raised by approximately 5 mV (equivalent to 250 V at the input pins). 40LOG (f2/f1) f1 f2 f2 2/f 1 FREQUENCY A 1-POLE FILTER, CORNER f1, AND A 2-POLE FILTER, CORNER f2, HAVE THE SAME ATTENUATION –40LOG (f2/f1) AT FREQUENCY f2 2/f 1 20dB/DECADE 40dB/DECADE Figure 10. Comparative Responses of 1-Pole and 2-Pole Low-Pass Filters |
Codice articolo simile - AD8200_15 |
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Descrizione simile - AD8200_15 |
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