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AD7621ACPZ1 Fiches technique(PDF) 19 Page - Analog Devices |
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AD7621ACPZ1 Fiches technique(HTML) 19 Page - Analog Devices |
19 / 32 page AD7621 Rev. 0 | Page 19 of 32 External Reference (PDBUF = High, PRBUF = High) To use an external reference directly on the REF pin, PDREF and PDBUF should both be high. For improved drift performance, an external reference, such as the AD780 or ADR431, can be used. The advantages of directly using the external voltage reference are: • SNR and dynamic range improvement (about 1.7 dB) resulting from the use of a reference voltage very close to the supply (2.5 V) instead of a typical 2.048 V reference when the internal reference is used. This is calculated by ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = 50 . 2 048 . 2 log 20 SNR • Power savings when the internal reference is powered down (PBREF = PDBUF = high). PDREF and PDBUF power down the internal reference and the internal reference buffer, respectively. Reference Decoupling Whether using an internal or external reference, the AD7621 voltage reference input (REF) has a dynamic input impedance; therefore, it should be driven by a low impedance source with efficient decoupling between the REF and REFGND inputs. This decoupling depends on the choice of the voltage reference, but usually consists of a low ESR capacitor connected to REF and REFGND with minimum parasitic inductance. A 10 μF (X5R, 1206 size) ceramic chip capacitor (or 47 μF tantalum capacitor) is appropriate when using either the internal reference or one of these recommended reference voltages: • The low noise, low temperature drift ADR431 and AD780 • The low power ADR291 • The low cost AD1582 The placement of the reference decoupling is also important to the performance of the AD7621. The decoupling capacitor should be mounted on the same side as the ADC right at the REF pin with a thick PCB trace. The REFGND should also connect to the reference decoupling capacitor with the shortest distance. For applications that use multiple AD7621 devices, it is more effective to use the internal reference buffer in order to buffer the reference voltage. The voltage reference temperature coefficient (TC) directly impacts full scale; therefore, in applications where full-scale accuracy matters, care must be taken with the TC. For instance, a ±15 ppm/°C TC of the reference changes full-scale by ±1 LSB/°C. Temperature Sensor The TEMP pin measures the temperature of the AD7621. To improve the calibration accuracy over the temperature range, the output of the TEMP pin is applied to one of the inputs of the analog switch (such as, ADG779), and the ADC itself is used to measure its own temperature. This configuration is shown in Figure 28. ADG779 AD8021 CC ANALOG INPUT (UNIPOLAR) AD7621 IN+ TEMPERATURE SENSOR TEMP Figure 28. Use of the Temperature Sensor POWER SUPPLY The AD7621 uses three sets of power supply pins: an analog 2.5 V supply AVDD, a digital 2.5 V core supply DVDD, and a digital input/output interface supply OVDD. The OVDD supply allows direct interface with any logic working between 2.3 V and 5.25 V. To reduce the number of supplies needed, the digital core (DVDD) can be supplied through a simple RC filter from the analog supply as shown in Figure 23. Power Sequencing The AD7621 is independent of power supply sequencing once OVDD does not exceed DVDD by more than 0.3 V until DVDD = 2.3 V during any time; for instance, at power-up or power-down (see the Absolute Maximum Ratings section). Additionally, it is very insensitive to power supply variations over a wide frequency range as shown in Figure 29. FREQUENCY (kHz) 45 75 70 65 60 55 50 1 10 100 1k 10k EXT REF INT REF Figure 29. PSRR vs. Frequency |
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Description similaire - AD7621ACPZ1 |
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