Using the AD7328 8-Channel ADC in Single-Ended Applications
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The document discusses using the AD7328 8-channel ADC in single-ended applications. It provides an overview of the AD7328 and describes its analog input configuration options of single-ended, true differential, and pseudo differential. It recommends using an operational amplifier driver to ensure maximum performance and discusses design considerations for the analog input driver and reference voltage source.
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Using the AD7328 8-Channel ADC in Single-Ended Applications
- 9. Single-Ended Configuration Verified Circuits are designed for ease of use and proven to work by ADI. The AD797 is configured as a buffer to drive the AD7328.
- 12. Converter Operation Figure 1 Figure 2 Figure 3 Figure 4
Editor's Notes
- Welcome to the training module on Analog Devices Using the AD7328 8-Channel ADC in Single- Ended Applications.
- The AD7328 is a fast, 8-channel, 12-bit plus sign, bipolar input, serial A/D converter. The AD7328 can accept true bipolar analog input signals. It has four software-selectable input ranges, ±10 V, ±5 V, ±2.5 V, and 0 V to +10 V. Each analog input channel can be independently programmed to one of the four input ranges. The analog input channels on the AD7328 can be programmed to be single-ended, true differential, or pseudo differential. The ADC contains a 2.5 V internal reference, but it also allows for external reference operation. The ADC has a high speed serial interface that can operate at throughput rates up to 1 MSPS.
- The figure shows a typical connection diagram for the AD7328. In this configuration, the AGND pin is connected to the analog ground plane of the system, and the DGND pin is connected to the digital ground plane of the system. The AD7328 is configured to operate with the internal 2.5V reference. A 680nF decoupling capacitor is required when operating with the internal reference. The VCC pin can be connected to either a 3 V or 5 V supply voltage. The V DD and V SS are the dual supplies for the high voltage analog input structures. The voltage on these pins must be equal to or greater than the highest analog input range selected on the analog input channels. The V DRIVE pin is connected to the supply voltage of the microprocessor. The voltage applied to the V DRIVE input controls the voltage of the serial interface. The analog inputs on the AD7328 can be configured to operate in single-ended, true differential, or pseudo differential mode. We will discuss the analog input configurations in next few pages.
- The eight analog inputs of the AD7328 can be programmed to be three different modes. When operating in single-ended mode, the AD7328 has a total of eight analog inputs. Each of them can be independently programmed to one of the four analog input ranges. The AD7328 can be configured as four true differential analog input pairs. Differential signals have some benefits over single-ended signals, including better noise immunity based on the device’s common-mode rejection and improvements in distortion performance. The amplitude of the differential signal is the difference between the signals applied to the V IN+ and V IN− pins in each differential pair. The AD7328 can have four pseudo differential pairs or seven pseudo differential inputs referenced to a common V IN− pin. In this configuration, the V IN+ inputs are coupled to the signal source and a DC input is applied to the V IN− pin. Pseudo differential inputs separate the analog input signal ground from the ADC ground, allowing cancellation of dc common-mode voltages.
- The AD7328 can accept true bipolar input signals. On power-up, the analog inputs operate as eight single-ended analog input channels. If true differential or pseudo differential is required, a write to the control register is necessary after power-up to change this configuration. The figures show the equivalent analog input circuit of the AD7328 in single-end mode and differential mode. The two diodes provide ESD protection for the analog inputs. Resistor R1 is a lumped component made up of the on-resistance of the input multiplexer and the track-and-hold switch. Capacitor C2 is the sampling capacitor. The track-and-hold on the analog input of the AD7328 allows the ADC to accurately convert an input sine wave of full-scale amplitude to 13-bit accuracy.
- In applications where the harmonic distortion and signal-to-noise ratio are critical specifications, the analog input of the AD7328 should be driven from a low impedance source. Large source impedances significantly affect the AC performance of the ADC and can necessitate the use of an input buffer amplifier. When no amplifier is used to drive the analog input, the source impedance should be limited to low values. Due to the programmable nature of the analog inputs on the AD7328, the choice of op amp used to drive the inputs is a function of the particular application and depends on the input configuration and the analog input voltage ranges selected. The driver amplifier must be able to settle for a full-scale step to a 13-bit level, 0.0122%, in less than the specified acquisition time of the AD7328.
- Typically for voltage input ADCs, a number of ADC input formats exist including single-ended and differential inputs. Single-ended input is a widely used technique and is the simplest method for transmitting analog voltage over inputs. In single-ended applications, one input carries a varying voltage that represents the signal, while another input is connected to a reference voltage, usually ground. The primary advantage of using single-ended inputs is that a minimal number of connections are needed to transmit multiple signals. Single-ended inputs are ideal if the signal source and ADC are close to each other. But single-ended inputs are more susceptible to coupled-noise and DC offsets. However, signal conditioning circuitry can reduce these effects. In differential applications, two wires are routed in parallel, and sometimes twisted together so that both wires receive the same interference. In this configuration, one wire carries the signal while the second wire carries the inverse of the signal. Advantages of differential signaling include noise reduction by rejecting common-mode interference and greater dynamic range.
- This figure shows a typical connection diagram when operating the ADC in single-ended mode, where the AD797 is used to buffer the signal before applying it to the ADC analog inputs. In this configuration, the ADC will have 8 single-ended analog inputs.
- Differential operation requires that V IN+ and V IN− be simultaneously driven with two signals of equal amplitude that are 180° out of phase. The common mode must be set up externally to the AD7328. The common-mode range is determined by the REFIN/OUT voltage, the VCC supply voltage, and the particular amplifier used to drive the analog inputs. Differential mode with either an AC input or a DC input provides the best THD performance over a wide frequency range. Because not all applications have a signal preconditioned for differential operation, there is often a need to perform a single-ended to differential conversion. This single-ended-to-differential conversion can be performed using an op amp pair. The figure shows how an AD8620 op-amp can be used to convert a single ended into a differential signal which can be applied to the AD7328 analog inputs, so that the AD7328 can be used in single-ended application with full differential operation. This configuration ensures the output impedances of the amplifier driving the V IN+ and V IN− pins are matched; otherwise, the two inputs of the ADC have different settling times, resulting in errors.
- The AD7328 can operate with either the internal 2.5 V on-chip reference or an externally applied reference. The internal reference is selected by setting the Ref bit in the control register to 1. On power-up, the Ref bit is 0, which selects the external reference for the AD7328 conversion. The internal reference circuitry consists of a 2.5V band gap reference and a reference buffer. The AD7328 allows for an external reference voltage to be applied to the REFIN/REFOUT pin. The specified voltage input range on the reference voltage is from 2.5V to 3V. Using 3V reference voltages instead of 2.5V allows the AD7328 to accept larger input signals. On both verified circuits the AD780 is used as an external reference source. The AD780 is a 2.5 V/3 V ultra-high precision voltage reference, which enables flexibility in the voltage range selected.
- The AD7328 is a successive approximation ADC built around two capacitive DACs. Figure 1 and Figure 2 show simplified schematics of the ADC in single-ended mode during the acquisition and conversion phases, respectively. Figure 3 and Figure 4 show simplified schematics of the ADC in differential mode during acquisition and conversion phases, respectively. In Figure 1, SW2 is closed and SW1 is in Position A, the comparator is held in a balanced condition, and the sampling capacitor array acquires the signal on the input. When the ADC starts a conversion (Figure 3), SW2 opens and SW1 moves to Position B, causing the comparator to become unbalanced. The control logic and the charge redistribution DAC are used to add and subtract fixed amounts of charge from the capacitive DAC 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.
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