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ADS822 Datasheet(PDF) 9 Page - Burr-Brown (TI) |
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ADS822 Datasheet(HTML) 9 Page - Burr-Brown (TI) |
9 / 12 page 9 ® ADS822, ADS825 FIGURE 2. AC-Coupled Input Configuration for a 2Vp-p Full-Scale Range and a Common-Mode Voltage, VCM, at +2.5V Derived From the Internal Top (REFT) and Bottom Reference (REFB). FIGURE 3. AC-Coupling the Dual Supply Amplifier OPA642 to the ADS822 for a 2Vp-p Full-Scale Input Range. For applications requiring the driving amplifier to provide a signal amplification, with a gain ≥ 5, consider using decom- pensated voltage-feedback op amps, like the OPA643, or current-feedback op amps like the OPA681 and OPA658. DC-coupled with Level Shift Several applications may require that the bandwidth of the signal path include DC, in which case, the signal has to be DC-coupled to the A/D converter. In order to accomplish this, the interface circuit has to provide a DC level shift to the analog input signal. The circuit shown in Figure 4 employs a dual op amp, A1, to drive the input of the ADS822 and ADS825, and level shifts the signal to be compatible with the selected input range. With the RSEL pin tied to the supply and the INT/EXT pin to ground, the ADS822 and ADS825 are configured for a 2Vp-p input range and use the internal references. The complementary input (IN) may be appropriately biased using the +2.5V common-mode voltage available at the CM pin. One half of amplifier A1 buffers the REFB pin and drives the voltage divider R1, R2. Due to the op amp’s noise gain of +2V/V, assuming RF = RIN , the common-mode voltage (VCM) has to be re-scaled to +1.25V. This results in the correct DC level of +2.5V for the signal input (IN). Any DC voltage differences between the IN and IN inputs of the ADS822 and ADS825 effectively produces an offset, which can be corrected for by adjusting the resistor values of the divider, R1and R2. The selection criteria for a suitable op amp should include the supply voltage, input bias current, output voltage swing, distortion, and noise specification. Note that in this example, the overall signal phase is in- verted. To re-establish the original signal polarity, it is always possible to interchange the IN and IN connections. +V IN 0V –V IN OPA680 V IN R F 402 Ω 1.62k Ω R G 402 Ω ADS822 ADS825 R S 50 Ω 10pF 0.1 µF IN IN CM INT/EXT GND REFT +3.5V 1.62k Ω 50 Ω V CM +2.5V REFB +1.5V 0.1 µF 0.1 µF RSEL +V S +5V +5V OPA642 V IN R F 402 Ω 1.62k Ω R G 402 Ω ADS822 ADS825 R S 24.9 Ω 1.62k Ω 100pF 0.1 µF 0.1 µF IN IN CM REFB +1.5V INT/EXT GND REFT +3.5V RSEL +V S +5V +5V –5V |
Similar Part No. - ADS822 |
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Similar Description - ADS822 |
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