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ADS5120CGHK Datasheet(PDF) 11 Page - Texas Instruments |
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ADS5120CGHK Datasheet(HTML) 11 Page - Texas Instruments |
11 / 18 page ADS5120 11 SBAS247B www.ti.com For the single-ended mode, the signal is applied to one of the inputs while the other input is biased with a DC voltage to the required common-mode level. Both inputs are identical in terms of their impedance and performance. Applying the signal to the complementary input (AIN–) instead of the AIN+ input, however, will invert the orientation of the input signal relative to the output code. This could be helpful, for ex- ample, if the input driver operates in inverting mode using input AIN– as the signal input will restore the phase of the signal to its original orientation. INPUT DRIVER CONFIGURATIONS Transformer-Coupled Interface If the application requires a signal conversion from a single- ended source to drive the ADS5120 differentially, a RF- transformer might be a good solution. The selected trans- former must have a center tap in order to apply the common- mode DC voltage necessary to bias the converter inputs. AC- grounding the center tap will generate the differential signal swing across the secondary winding. Consider a step-up transformer to take advantage of signal amplification without the introduction of another noise source. Furthermore, the reduced signal swing from the source may lead to an im- proved distortion performance. The differential input configuration may provide a noticeable advantage of achieving good SFDR performance over a wide range of input frequencies. In this mode, both inputs (AIN+ and AIN–) of the ADS5120 see matched impedances. Figure 2 shows the schematic for the suggested transformer- coupled interface circuit. The component values of the R-C low-pass may be optimized depending on the desired roll-off frequency. Single-Ended, AC-Coupled Driver The circuit of Figure 3 shows an example for driving the inputs of the ADS5120 in a single-ended configuration. The signal is AC-coupled between the driver amplifier and the converter input (AIN+). This allows for setting the required common-mode voltages for the ADC and op amp separately. The single-supply op amp is biased at mid-supply by two resistors connected at its noninverting input. Connecting each input to the CML pin provides the required common- mode voltage for the inputs of the ADS5120. Here, two resistors of equal value ensure that the inputs see closely matched source impedances. If the op amp features a disable function, it could be easily tied together with the power-down pin of the ADS5120 channel (STBY). In the circuit example depicted in Figure 3, the OPA355’s EN pin is FIGURE 2. Converting a Single-Ended Input Signal into a Differential Signal Using a RF-Transformer. R IN R IN C IN 0.1 µF R T 0.1 µF 1:n R S OPA690 R 1 R 2 V IN ADS5120 AIN+ AIN– CML –5V +5V OPA355 V IN R F 604 Ω R G 604 Ω 1.82k Ω 1.82k Ω ADS5120 R S 24 Ω EN 18pF 0.1 µF 0.1 µF R 1 AIN+ STBY CML AIN– 0.1 µF 0.1 µF R 2 +3V/+5V FIGURE 3. Single-Ended, AC-Coupled Driver Configuration for Single Supply. |
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