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ADS830 Datasheet(PDF) 10 Page - Burr-Brown (TI) |
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ADS830 Datasheet(HTML) 10 Page - Burr-Brown (TI) |
10 / 11 page 10 ® ADS830 REFT +3.0V ADS830 CMV +2.5V REFB +2.0V R 1 1k Ω R 2 1k Ω 0.1µF 0.1µF 2.2µF + 2.2µF + the resistive reference ladder. The bandgap reference circuit includes logic functions that allow to set the analog input swing of the ADS830 to either a 1Vp-p or 2Vp-p full-scale range simply by tying the RSEL pin to a LOW or HIGH potential, respectively. While operating the ADS830 in the external reference mode, the buffer amplifiers for REFT and REFB are disconnected from the reference ladder. As shown, the ADS830 has internal 50k Ω pull-up resistors at the Range Select pin (RSEL) and Reference Select pin (INT/EXT). Leaving those pins open configures the ADS830 for a 2Vp-p input range and external reference operation. Setting the ADS830 up for internal reference mode requires to bring the INT/EXT pin LOW. The reference buffers can be utilized to supply up to 1mA (sink and source) to external circuitry. To ensure proper operation with any reference configurations, it is necessary to provide solid bypassing at the reference pins in order to keep the clock feedthrough to a minimum (Figure 6). All bypassing capacitors should be located as close to their respective pins as possible. FIGURE 8. Configuration Example for External Reference Operation. The common-mode voltage available at the CM pin may be used as a bias voltage to provide the appropriate offset for the driving circuitry. However, care must be taken not to appreciably load this node, which is not buffered and has a high impedance. An alternative way of generating a com- mon-mode voltage is given in Figure 7. Here, two external precision resistors (1% tolerance or better) are located between the top and bottom reference pins. The common- mode voltage, CMV, will appear at the midpoint. EXTERNAL REFERENCE OPERATION For even more design flexibility, the internal reference can be disabled and an external reference voltage be used. The utilization of an external reference may be considered for applications requiring higher accuracy, improved tempera- ture performance, or a wide adjustment range of the converter’s full-scale range. Especially in multichannel applications, the use of a common external reference has the benefit of obtaining better matching of the full-scale range between converters. The external references can vary as long as the value of the external top reference REFTEXT stays within the range of (VS – 1.25V) and (REFB + 0.8V), and the external bottom reference REFBEXT stays within 1.25V and (REFT – 0.8V), see Figure 8. The full-scale input signal range (FSR) of the ADS830 is determined by the voltage difference across the reference pins REFT and REFB (FSR = REFT – REFB), while the common-mode voltage is defined by CMV = (REFT + REFB)/2. In order to maintain good ac performance, it is recommended that the typical common-mode voltage be kept at +2.5V while setting the external reference voltages. It is possible, however, to deviate from this common-mode level without significantly impacting the performance. In particular, DC-coupled applications may benefit from a ADS830 IN IN INT/EXT REFT GND REFB External Top Reference REFT = REFB +0.8V to +3.75V +VS BA RSEL GND +5V External Bottom Reference REFB = REFT –0.8V to +1.25V V IN A - Short for 1Vp-p Input Range B - Short for 2Vp-p Input Range (Default) CMV FIGURE 7. Alternative Circuit to Generate Common-Mode Voltage. |
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