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ADS7821U Arkusz danych(PDF) 9 Page - Burr-Brown (TI) |
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ADS7821U Arkusz danych(HTML) 9 Page - Burr-Brown (TI) |
9 / 10 page 9 ® ADS7821 INPUT RANGE The ADS7821 offers a standard 0V to 5V input range. Figure 6 shows the required circuit connections for the ADS7821 with and without the gain adjustment hardware. Adjustments for offset and gain are described in the calibra- tion section of this data sheet. CALIBRATION The ADS7821 can be trimmed in hardware or software. There is no external offset adjustment. If offset adjustment is re- quired, an op amp featuring an offset trim pin should be used to drive the ADS7821. The offset should be trimmed before the gain since the offset directly affects the gain. To achieve optimum performance, several iterations may be required. GAIN ADJUSTMENT To calibrate the gain of the ADS7821, a 576k Ω resistor can be tied between the REF pin and a 50k Ω potentiometer as shown in Figure 6a. The calibration range is ±15mV for the gain. REFERENCE The ADS7821 can operate with its internal 2.5V reference or an external reference. By applying an external reference to pin 3, the internal reference can be bypassed. The reference voltage at REF is buffered internally with the output on CAP (pin 4). REF REF (pin 3) is an input for an external reference or the output for the internal 2.5V reference. A 2.2 µF capacitor should be connected as close to the REF pin as possible. The capacitor and the output resistance of REF create a low pass filter to bandlimit noise on the reference. Using a smaller value capacitor will introduce more noise to the reference degrad- ing the SNR and SINAD. The REF pin should not be used to directly drive external loads. The range for the external reference is 2.3V to 2.7V and determines the actual LSB size. Increasing the reference voltage will increase the full scale range and the LSB size of the converter which can improve the SNR. CAP CAP (pin 4) is the output of the internal reference buffer. A 2.2 µF capacitor should be placed as close to the CAP pin as possible to provide optimum switching currents for the CDAC throughout the conversion cycle and compensation for the output of the internal buffer. Using a capacitor any smaller than 1 µF can cause the output buffer to oscillate and may not have sufficient charge for the CDAC. Capacitor values larger than 2.2 µF will have little affect on improving performance. The output of the buffer is capable of driving up to 2mA of current to a static load. Static loads requiring more than 2mA of current from the CAP pin will begin to degrade the linearity of the ADS7821. Use of an external buffer is recommended for loads requiring more than 2mA. Do not attempt to directly drive any dynamic load with the output voltage on CAP. This will cause performance degradation of the converter. a) With Hardware Gain Trim b) Without Hardware Gain Trim FIGURE 6. Circuit Diagram With and Without External Gain Trim. 1 2 3 4 5 AGND2 CAP REF AGND1 V IN + 2.2µF 0 to +5V + 2.2µF 1 2 3 4 5 AGND2 NOTE: Use 1% metal film resistors. CAP REF AGND1 V IN + 2.2µF +5V 50k Ω 576k Ω + 2.2µF Gain 0 to +5V |
Podobny numer części - ADS7821U |
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Podobny opis - ADS7821U |
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