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ADS7804UB Datasheet(PDF) 9 Page - Burr-Brown (TI) |
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ADS7804UB Datasheet(HTML) 9 Page - Burr-Brown (TI) |
9 / 10 page ® ADS7804 9 FIGURE 7. Full Scale Transfer Function. REFERENCE The ADS7804 can operate with its internal 2.5V reference or an external reference. By applying an external reference to pin 5, the internal reference can be bypassed. The reference voltage at REF is buffered internally with the output on CAP (pin 4). The internal reference has an 8 ppm/ °C drift (typical) and accounts for approximately 20% of the full scale error (FSE = ±0.5% for low grade, ±0.25% for high grade). 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 drive external AC or DC 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 DC load. DC loads requiring more than 2mA of current from the CAP pin will begin to degrade the linearity of the ADS7804. Using an external buffer will allow the internal reference to be used for larger DC loads and AC loads. Do not attempt to directly drive an AC load with the output voltage on CAP. This will cause performance degra- dation of the converter. –9.99983V –9.9998V –10V Digital Output 7FF Analog Input 800 9.999815V +10V Ideal Transfer Function With External Resistors Range of Transfer Function Without External Resistors 9.9997V –50mV –15mV |
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