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AD7356BRUZ Datasheet(PDF) 13 Page - Analog Devices |
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AD7356BRUZ Datasheet(HTML) 13 Page - Analog Devices |
13 / 21 page AD7356 Data Sheet Rev. B | Page 12 of 20 THEORY OF OPERATION CIRCUIT INFORMATION The AD7356 is a high speed, dual, 12-bit, single-supply, successive approximation analog-to-digital converter (ADC). The device operates from a 2.5 V power supply and features throughput rates of up to 5 MSPS. The AD7356 contains two on-chip differential track-and-hold amplifiers, two successive approximation ADCs, and a serial interface with two separate data output pins. The part is housed in a 16-lead TSSOP, offering the user considerable space-saving advantages over alternative solutions. The serial clock input accesses data from the part but also provides the clock source for each successive approximation ADC. The AD7356 has an on-chip 2.048 V reference. If an external reference is desired the internal reference can be overdriven with a reference value ranging from (2.048 V + 100 mV) to VDD. If the internal reference is to be used elsewhere in the system, then the reference output needs to be buffered first. The differential analog input range for the AD7356 is VCM ± VREF/2. The AD7356 features power-down options to allow power saving between conversions. The power-down feature is implemented via the standard serial interface, as described in the Modes of Operation section. CONVERTER OPERATION The AD7356 has two successive approximation ADCs, each based around two capacitive DACs. Figure 14 and Figure 15 show simplified schematics of one of these ADCs in acquisition and conversion phase. The ADC comprises a control logic, a SAR, and two capacitive DACs. In Figure 14 (the acquisition phase), SW3 is closed, SW1 and SW2 are in Position A, the comparator is held in a balanced condition, and the sampling capacitor arrays acquire the differential signal on the input. CAPACITIVE DAC CAPACITIVE DAC CONTROL LOGIC COMPARATOR SW3 SW1 A A B B SW2 CS CS VIN+ VIN– VREF Figure 14. ADC Acquisition Phase When the ADC starts a conversion (see Figure 15), SW3 opens and SW1 and SW2 move to Position B, causing the comparator to become unbalanced. Both inputs are disconnected once the conversion begins. The control logic and charge redistribution DACs are used to add and subtract fixed amounts of charge from the sampling capacitor arrays to bring the comparator back into a balanced condition. When the comparator is rebalanced, the conversion is complete. The control logic generates the ADC output code. The output impedances of the sources driving the VIN+ and VIN− pins must be matched; otherwise, the two inputs may have different settling times, resulting in errors. CAPACITIVE DAC CAPACITIVE DAC CONTROL LOGIC COMPARATOR SW3 SW1 A A B B SW2 CS CS VIN+ VIN– VREF Figure 15. ADC Conversion Phase ANALOG INPUT STRUCTURE Figure 16 shows the equivalent circuit of the analog input structure of the AD7356. The four diodes provide ESD protection for the analog inputs. Care must be taken to ensure that the analog input signals never exceed the supply rails by more than 300 mV. This causes these diodes to become forward biased and start conducting into the substrate. These diodes can conduct up to 10 mA without causing irreversible damage to the part. The C1 capacitors in Figure 16 are typically 8 pF and can primarily be attributed to pin capacitance. The R1 resistors are lumped components made up of the on resistance of the switches. The value of these resistors is typically about 30 Ω. The C2 capacitors are the sampling capacitors of the ADC with a capacitance of 32 pF typically. VDD C1 D D VIN+ R1 C2 VDD C1 D D VIN– R1 C2 Figure 16. Equivalent Analog Input Circuit, Conversion Phase–Switches Open, Track Phase—Switches Closed |
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