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ADS807E Datasheet(PDF) 11 Page - Texas Instruments |
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ADS807E Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 22 page  ADS807 11 SBAS072A www.ti.com AC-Coupled, Single-Ended-to-Differential Interface for Single-Supply Operation The previously discussed interface circuit can be modified if the system only allows for a single-supply operation, e.g., VS = +5V. Single-supply operation requires the driver ampli- fier to be biased as well in order to process a bipolar input signal. Typically, single-supply amplifiers do not achieve distortion performance as well as dual-supply op amps. The driver amplifier’s output swing must exceed the full-scale input range of the converter. In addition, dual op amps, such as the current-feedback OPA2681, should be considered since they provide the closest open-loop gain and phase matching between the two channels. Shown in Figure 3 is a single-supply interface circuit for an AC-coupled input signal. With the ADS807 set to the 2Vp-p input range, the top and bottom references (REFT, REFB) provide an output voltage of +3.0V and +2.0V, respectively. The CM output of the ADS807 is used to bias the inputs of the driving amplifiers. Using the OPA2681 on a single +5V supply, its ideal com- mon-mode point is +2.5V, which coincides with the recom- mended common-mode input level for the ADS807, thus obviating the need for coupling capacitors between the amplifiers and the converter. The addition of a small series resistor (RS) between the output of the op amps and the input of the ADS807 will be beneficial in almost all interface configurations. It will decouple the op amp’s output from the capacitive load and avoid gain peaking, which can result in increased noise. For best spurious and distortion performance, the resistor value should be kept below 100 Ω. Furthermore, the series resistor in combination with the shunt capacitor, establishes a passive low-pass filter limiting the bandwidth for the wideband noise, thus improving the SNR. The spurious-free dynamic range of this single-supply front end is limited by the 2nd-harmonic distortion. An improvement of several dB may be realized by adding a pull-down resistor (RP) at the output of each amplifier. This pulls a DC bias current out of the output stage of the amplifier. It is set to approximately 5mA in Figure 3, but will vary depending on the amplifier used. FIGURE 2. AC-Coupled Differential Driver Interface with OPA642. FIGURE 3. AC-Coupled, Differential Interface for Single-Supply Operation. V IN ADS807E 68pF IN CM IN +5V 68pF 0.1 µF 0.1 µF 0.1 µF A1 A2 OPA2681 R G 499 Ω R F 499 Ω 499 Ω R S 24.9 Ω V CM = +2.5V R P 499 Ω R P 499 Ω R F 499 Ω R IN 249 Ω R S 24.9 Ω V IN ADS807E 100pF IN IN 402 Ω 1.82k Ω 1.82k Ω 16.5 Ω 200 Ω 402 Ω 402 Ω REFB 100pF 0.1 µF 0.1 µF A1 OPA642 A2 OPA642 REFT 1.82k Ω 1.82k Ω 16.5 Ω |
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