Using the AD7328 8-Channel ADC in Single- Ended Applications <ul><li>Source: ANALOG DEVICES </li></ul>
Introduction <ul><li>Purpose </li></ul><ul><ul><li>This training module discuss the design to optimize the performance of ...
Overview of the AD7328 <ul><li>The AD7328 can accept true bipolar analog input signals, ±10 V, ±5 V, ±2.5 V, and 0 V to +1...
Typical Connection Diagram
Analog Inputs of the AD7328 <ul><li>Single-End Inputs </li></ul><ul><ul><li>The AD7328 has a total of eight analog inputs ...
Analog Input Structure <ul><li>On the AD7328, the bipolar analog signal is sampled directly onto the sampling capacitor.  ...
Driver Amplifier Choice <ul><li>The analog input of the AD7328 should be driven from a low impedance source. </li></ul><ul...
Single-Ended vs. Differential Applications <ul><li>In  single-ended applications , one wire carries a varying voltage that...
Single-Ended Configuration Verified Circuits  are designed for ease of use and proven to work by ADI.  The AD797 is config...
Single-Ended to Differential Conversion   <ul><li>The AD8620 is an ideal choice of op-amp which can be used to provide a s...
Reference Voltage Source <ul><li>On power-up, the external reference mode is the default condition. </li></ul><ul><li>The ...
Converter Operation Figure 1 Figure 2 Figure 3 Figure 4
Additional Resource <ul><li>For ordering the AD7328 and selected parts, please click the part list or </li></ul><ul><li>Ca...
Upcoming SlideShare
Loading in...5
×

Using the AD7328 8-Channel ADC in Single-Ended Applications

641

Published on

Discussion of the design to optimize the performance of the AD7328

Published in: Technology, Business
0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total Views
641
On Slideshare
0
From Embeds
0
Number of Embeds
1
Actions
Shares
0
Downloads
0
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide
  • 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.
  • Thank you for taking the time to view this presentation on “ Using the AD7328 8-Channel ADC in Single- Ended Applications” . If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simply call our sales hotline. For more technical information you may either visit the ADI site – link shown – or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.
  • Using the AD7328 8-Channel ADC in Single-Ended Applications

    1. 1. Using the AD7328 8-Channel ADC in Single- Ended Applications <ul><li>Source: ANALOG DEVICES </li></ul>
    2. 2. Introduction <ul><li>Purpose </li></ul><ul><ul><li>This training module discuss the design to optimize the performance of the AD7328. The selected operational amplifier (op-amp) and reference voltage source ensure that the maximum AD7328 performance is achieved. </li></ul></ul><ul><li>Outline </li></ul><ul><ul><li>Overview of the AD7328 ADC </li></ul></ul><ul><ul><li>The AD7328 analog inputs configuration </li></ul></ul><ul><ul><li>Single-ended application using AD7328 </li></ul></ul><ul><ul><li>Reference voltage source </li></ul></ul><ul><li>Content </li></ul><ul><ul><li>13 pages </li></ul></ul>
    3. 3. Overview of the AD7328 <ul><li>The AD7328 can accept true bipolar analog input signals, ±10 V, ±5 V, ±2.5 V, and 0 V to +10 V unipolar signals. </li></ul><ul><li>The eight analog inputs can be configured as eight single-ended inputs, four true differential input pairs, four pseudo differential inputs, or seven pseudo differential inputs. </li></ul><ul><li>1 MSPS serial interface. SPI®-/QSPI™-/DSP-/MICROWIRE™-compatible interface. </li></ul><ul><li>Low power, 30 mW, at a maximum throughput rate of 1 MSPS. </li></ul><ul><li>Channel sequencer. </li></ul>Analog Inputs
    4. 4. Typical Connection Diagram
    5. 5. Analog Inputs of the AD7328 <ul><li>Single-End Inputs </li></ul><ul><ul><li>The AD7328 has a total of eight analog inputs when operating in single-ended mode. </li></ul></ul><ul><ul><li>Each analog input can be independently programmed to one of the four analog input ranges. </li></ul></ul><ul><li>True Differential Mode </li></ul><ul><ul><li>The AD7328 can have four true differential analog input pairs. </li></ul></ul><ul><ul><li>Differential signals have better noise immunity based on the device’s common-mode rejection and improvements in distortion performance. </li></ul></ul><ul><li>Pseudo Differential Inputs </li></ul><ul><ul><li>The AD7328 can have four pseudo differential pairs or seven pseudo differential inputs referenced to a common V IN− pin. </li></ul></ul><ul><ul><li>The V IN+ inputs are coupled to the signal source and must have an amplitude within the selected range. </li></ul></ul>
    6. 6. Analog Input Structure <ul><li>On the AD7328, the bipolar analog signal is sampled directly onto the sampling capacitor. </li></ul><ul><li>This gives the AD7328 high analog input impedance. </li></ul><ul><li>As the sampling frequency is reduced, the analog input impedance further increases. </li></ul><ul><li>As the analog input impedance increases, the current required to drive the analog input therefore decreases. </li></ul>Equivalent Analog Input Circuit (Single-Ended) Equivalent Analog Input Circuit (Differential Mode) Z = 1/( f S × C S ) f S is the sampling frequency C S is the sampling capacitor value
    7. 7. Driver Amplifier Choice <ul><li>The analog input of the AD7328 should be driven from a low impedance source. </li></ul><ul><li>Large source impedances must use an input buffer amplifier. </li></ul><ul><li>When no amplifier is used to drive the analog input, the source impedance should be limited to low values. </li></ul><ul><li>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. </li></ul><ul><li>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. </li></ul>
    8. 8. Single-Ended vs. Differential Applications <ul><li>In single-ended applications , one wire carries a varying voltage that represents the signal, while another wire is connected to a reference voltage, usually ground. </li></ul><ul><li>In differential applications , two wires are routed in parallel, and sometimes twisted together so that both wires receive the same interference. </li></ul>
    9. 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.
    10. 10. Single-Ended to Differential Conversion <ul><li>The AD8620 is an ideal choice of op-amp which can be used to provide a single-ended to differential driver for AD7328. </li></ul><ul><li>The AD8620 is a precision, low input bias current, wide bandwidth JFET operational amplifier </li></ul>Noninverting signal Inverting signal Verified Circuits are designed for ease of use and proven to work by ADI.
    11. 11. Reference Voltage Source <ul><li>On power-up, the external reference mode is the default condition. </li></ul><ul><li>The specified reference voltage range is from 2.5V to 3V. </li></ul><ul><li>When a 3 V reference is selected, the input ranges are ±12 V, ±6 V, ±3 V, and 0 V to +12 V. </li></ul><ul><li>For these ranges, the V DD and V SS supply must be equal to or greater than the maximum analog input range selected. </li></ul>AD780
    12. 12. Converter Operation Figure 1 Figure 2 Figure 3 Figure 4
    13. 13. Additional Resource <ul><li>For ordering the AD7328 and selected parts, please click the part list or </li></ul><ul><li>Call our sales hotline </li></ul><ul><li>For additional inquires contact our technical service hotline </li></ul><ul><li>For more product information go to </li></ul><ul><ul><li>http://www.analog.com/en/content/CU_verified_circuits_CN0047/fca.html </li></ul></ul>

    ×