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ADS902E Datasheet(PDF) 9 Page - Burr-Brown (TI) |
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ADS902E Datasheet(HTML) 9 Page - Burr-Brown (TI) |
9 / 11 page 9 ADS902 ® Again, the input coupling capacitor C1 and resistor R1 form a high-pass filter. At the same time, the input impedance is defined by R1. Possible op amps for A1 are CLC452, EL2180 or LM6152. Depending on the selected amplifier, the use of a pull-up/pull-down resistor (RP), located directly at its output, may considerably improve its distortion performance. Resistor RS isolates the op amp’s output from the capacitive load to avoid gain peaking or even oscillation. It can also be used to establish a defined bandwidth to reduce the wideband noise. Its value is usually between 10 Ω and 100Ω. DC-COUPLED INTERFACE CIRCUIT Shown in Figure 5 is a single-supply, DC-coupled circuit which can be set in a gain of –1V/V or higher. Depending on the gain determined by RF/RIN, the divider ratio set by resistors R1 and R2 must be adjusted to yield the correct common-mode voltage for the ADS902. With a +5V supply, the nominal signal input range of the ADS902 is 1Vp-p, typically centered around the common-mode voltage of +2.75V. EXTERNAL REFERENCE The ADS902 requires an external top and bottom reference on pin 22 (REFT) and 24 (REFB). Internally those pins are connected through a resistor ladder, which has a nominal resistance of 4k Ω (±15%). In order to establish a correct voltage drop across the ladder the external reference circuit must be able to supply typically 250 µA of current. With this current the full-scale input range of the ADS902 is set between +2.25V and +3.25V. In general, the voltage drop across REFT and REFB determines the input full-scale range (FSR) of the ADS902. Equation (4) can be used to calculate the span. FSR = REFT - REFB (4) Depending on the application several options exist how to supply the external reference voltages to the ADS902 with- out degrading the typical performance. LOW-COST REFERENCE SOLUTION The easiest way to achieve the required reference voltages is to place the reference ladder of the ADS902 between the supply rails. Two additional resistors (RT, RB) are necessary to set the correct current through the ladder (see Figure 6). However, depending on the desired full-scale swing and supply voltage, different resistor values might be selected. A 1 V IN R F +V S R 1 0.1µF 2k Ω 2k Ω IN CM REFB External Bottom Reference REFT R S R IN R 2 22pF C 2 ADS902 External Top Reference FIGURE 5. DC-Coupled, Single-Supply Interface Circuit. 1k Ω 1k Ω LpBy REFT REFB LnBy +V S IN CM R T 7k Ω 1k Ω +2.25V R B 9k Ω 1k Ω 0.1µF 0.1µF 0.1µF 0.1µF 0.1µF +5V V IN ADS902 0.1µF +2.75V 1k Ω 0.1µF L1 10µF +3.25V FIGURE 6. Low Cost Solution to Supply External Reference Voltages. |
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Similar Description - ADS902E |
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