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AD7612 Datasheet(PDF) 22 Page - Analog Devices |
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AD7612 Datasheet(HTML) 22 Page - Analog Devices |
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22 / 32 page  AD7612 Data Sheet Rev. A | Page 22 of 32 Internal Reference (REF = 5 V) (PDREF = Low, PDBUF = Low) To use the internal reference, the PDREF and PDBUF inputs must be low. This enables the on-chip band gap reference, buffer, and TEMP sensor resulting in a 5.00 V reference on the REF pin. The internal reference is temperature-compensated to 5.000 V ±35 mV. The reference is trimmed to provide a typical drift of 3 ppm/°C. This typical drift characteristic is shown in Figure 19. External 2.5 V Reference and Internal Buffer (REF = 5 V) (PDREF = High, PDBUF = Low) To use an external reference with the internal buffer, PDREF should be high and PDBUF should be low. This powers down the internal reference and allows the 2.5 V reference to be applied to REFBUFIN producing 5 V on the REF pin. The internal ref- erence buffer is useful in multiconverter applications since a buffer is typically required in these applications. External 5 V Reference (PDREF = High, PDBUF = High) To use an external reference directly on the REF pin, PDREF and PDBUF should both be high. PDREF and PDBUF power down the internal reference and the internal reference buffer, respectively. For improved drift performance, an external ref- erence such as the ADR445 or ADR435 is recommended. Reference Decoupling Whether using an internal or external reference, the AD7612 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 22 μF (X5R, 1206 size) ceramic chip capacitor (or 47 μF tantalum capacitor) is appropriate when using either the internal reference or the ADR445/ADR435 external reference. The placement of the reference decoupling is also important to the performance of the AD7612. 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 and to the analog ground plane with several vias. For applications that use multiple AD7612 or other PulSAR devices, it is more effective to use the internal reference buffer to buffer the external 2.5 V 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 AD7612. 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 30. ADG779 AD8021 CC ANALOG INPUT (UNIPOLAR) AD7612 IN+ TEMPERATURE SENSOR TEMP Figure 30. Use of the Temperature Sensor POWER SUPPLIES The AD7612 uses five sets of power supply pins: AVDD: analog 5 V core supply VCC: analog high voltage positive supply VEE: high voltage negative supply DVDD: digital 5 V core supply OVDD: digital input/output interface supply Core Supplies The AVDD and DVDD supply the AD7612 analog and digital cores respectively. Sufficient decoupling of these supplies is required consisting of at least a 10 μF capacitor and 100 nF on each supply. The 100 nF capacitors should be placed as close as possible to the AD7612. To reduce the number of supplies needed, the DVDD can be supplied through a simple RC filter from the analog supply, as shown in Figure 27. High Voltage Supplies The high voltage bipolar supplies, VCC and VEE are required and must be at least 2 V larger than the maximum input, VIN. For example, if using the bipolar 10 V range, the supplies should be ±12 V minimum. Sufficient decoupling of these supplies is also required consisting of at least a 10 μF capacitor and 100 nF on each supply. For unipolar operation, the VEE supply can be grounded with some slight THD performance degradation. Digital Output Supply The OVDD supplies the digital outputs and allows direct interface with any logic working between 2.3 V and 5.25 V. OVDD should be set to the same level as the system interface. Sufficient decou- pling is required consisting of at least a 10 μF capacitor and 100 nF with the 100 nF placed as close as possible to the AD7612. |
Similar Part No. - AD7612_15 |
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Similar Description - AD7612_15 |
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