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ADS8588S Datasheet(PDF) 30 Page - Texas Instruments |
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ADS8588S Datasheet(HTML) 30 Page - Texas Instruments |
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30 / 63 page  ADC 2.5 V VREF REFCAPB REFIN / REFOUT AGND 10 PF REFGND 10 PF REFSEL AVDD REFCAPA DVDD 30 ADS8588S, ADS8586S, ADS8584S SBAS642 – DECEMBER 2016 www.ti.com Product Folder Links: ADS8588S ADS8586S ADS8584S Submit Documentation Feedback Copyright © 2016, Texas Instruments Incorporated In oversampling mode (see the Oversampling Mode of Operation section), the ADC takes the first sample for each channel at the rising edge of the CONVSTA, CONVSTB signals. After converting the first sample, the subsequent samples are taken by an internally generated sampling control signal. The samples are then averaged to reduce the noise of the signal chain as well as to improve the SNR of the ADC. The final output is also decimated to provide a 16-bit output for each channel. Table 1 lists the typical SNR performance for both the ±10-V and ±5-V input ranges, including the –3-dB bandwidth and proportional maximum throughput per channel. When the oversampling ratio increases, there is a proportional improvement in the SNR performance and decrease in the bandwidth of the input filter. 8.3.8 Reference The ADS8588S, ADS8586S, and ADS8584S can operate with either an internal voltage reference or an external voltage reference using an internal gain amplifier. The internal or external reference selection is determined by an external REFSEL pin, as explained in the REFSEL (Input) section. The REFIN/REFOUT pin outputs the internal band-gap voltage (in internal reference mode) or functions as an input to the external reference voltage (in external reference mode). In both cases, the on-chip amplifier is always enabled. Use this internal amplifier to gain the reference voltage and drive the actual reference input of the internal ADC core for maximizing performance. The REFCAPA (pin 45) and REFCAPB (pin 44) pins must be shorted together externally and a ceramic capacitor of 10 µF (minimum) must be connected between this node and REFGND (pin 43) to ensure that the internal reference buffer is operating as closed loop. 8.3.8.1 Internal Reference The devices have an internal 2.5-V (nominal value) band-gap reference. In order to select the internal reference, the REFSEL pin must be tied high or connected to DVDD. When the internal reference is used, REFIN/REFOUT (pin 42) becomes an output pin with the internal reference value. A 10-μF (minimum) decoupling capacitor is recommended to be placed between the REFIN/REFOUT pin and REFGND (pin 43), as shown in Figure 57. The capacitor must be placed as close to the REFIN/REFOUT pin as possible. The output impedance of the internal band-gap creates a low-pass filter with this capacitor to band-limit the noise of the band-gap output. The use of a smaller capacitor increases the reference noise in the system, thus degrading SNR and SINAD performance. Do not use the REFIN/REFOUT pin to drive external ac or dc loads because of the limited current output capability of the pin. The REFIN/REFOUT pin can be used as a reference source if followed by a suitable op amp buffer. Figure 57. Device Connections for Using an Internal 2.5-V Reference |
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