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ADS800E Datasheet(PDF) 11 Page - Texas Instruments |
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ADS800E Datasheet(HTML) 11 Page - Texas Instruments |
11 / 23 page ADS800 11 SBAS035B www.ti.com APPLICATIONS DRIVING THE ADS800 The ADS800 has a differential input with a common-mode of +2.25V. For AC-coupled applications, the simplest way to create this differential input is to drive the primary winding of a transformer with a single-ended input. A differential output is created on the secondary if the center tap is tied to the common-mode voltage of +2.25V, as per Figure 5. This transformer-coupled input arrangement provides good high- frequency AC performance. It is important to select a trans- former that gives low distortion and does not exhibit core saturation at full-scale voltage levels. Since the transformer does not appreciably load the ladder, there is no need to buffer the Common-Mode (CM) output in this instance. In general, it is advisable to keep the current draw from the CM output pin below 0.5 µA to avoid nonlinearity in the internal reference ladder. A FET input operational amplifier such as the OPA130 can provide a buffered reference for driving external circuitry. The analog IN and IN inputs should be bypassed with 22pF capacitors to minimize track-and-hold glitches and to improve high input frequency performance. Figure 6 illustrates another possible low-cost interface circuit which utilizes resistors and capacitors in place of a trans- former. Depending on the signal bandwidth, the component values should be carefully selected in order to maintain the FIGURE 6. AC-Coupled Differential Input Circuit. product performance. The input capacitors, CIN, and the input resistors, RIN, create a high-pass filter with the lower corner frequency at fC = 1/(2pRINCIN). The corner frequency can be reduced by either increasing the value of RIN or CIN. If the circuit operates with a 50 Ω or 75Ω impedance level, the resistors are fixed and only the value of the capacitor can be increased. Usually, AC-coupling capacitors are electrolytic or tantalum capacitors with values of 1 µF or higher. It should be noted that these large capacitors become inductive with increased input frequency, which could lead to signal ampli- tude errors or oscillation. To maintain a low AC-coupling impedance throughout the signal band, a small value (e.g. 1 µF) ceramic capacitor could be added in parallel with the polarized capacitor. Capacitors CSH1 and CSH2 are used to minimize current glitches resulting from the switching in the input track-and- hold stage and to improve signal-to-noise performance. These capacitors can also be used to establish a low-pass filter and effectively reduce the noise bandwidth. In order to create a real pole, resistors RSER1 and RSER2 were added in series with each input. The cutoff frequency of the filter is deter- mined by fC = 1/(2pRSER • (CSH + CADC)) where RSER is the resistor in series with the input, CSH is the external capacitor from the input to ground, and CADC is the internal input capacitance of the A/D converter (typically 4pF). Resistors R1 and R2 are used to derive the necessary common-mode voltage from the buffered top and bottom references. The total load of the resistor string should be selected so that the current does not exceed 1mA. Although the circuit in Figure 6 uses two resistors of equal value so that the common-mode voltage is centered between the top and bottom reference (+2.25V), it is not necessary to do so. In all cases the center point, VCM, should be bypassed to ground in order to provide a low-impedance AC ground. If the signal needs to be DC coupled to the input of the ADS800, an operational amplifier input circuit is required. In the differential input mode, any single-ended signal must be modi- fied to create a differential signal. This can be accomplished by FIGURE 5. AC-Coupled Single-Ended to Differential Drive Circuit Using a Transformer. Mini-Circuits T T1-6-KK81 or equivalent 22 26 27 CM IN IN ADS800 AC Input Signal 22pF 22pF 0.1 µF ADS8xx R SER1 (1) 49.9 Ω R 3 1k Ω R 2 (6k Ω) R 1 (6k Ω) C 2 0.1 µF C SH1 22pF C SH2 22pF C 3 0.1 µF C 1 0.1 µF C IN 0.1 µF V CM C IN 0.1 µF R IN1 25 Ω R IN2 25 Ω R SER2 (1) 49.9 Ω +3.25V Top Reference +1.25V Bottom Reference IN NOTE: (1) Indicates optional component. IN |
Número de pieza similar - ADS800E |
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Descripción similar - ADS800E |
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