Converting a Single-Ended Signal with the AD7984 Differential PulSAR ADC

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Discussion of the reference design converting a single-ended input signal to a differential signal for use with the AD7984

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Converting a Single-Ended Signal with the AD7984 Differential PulSAR ADC

  1. 1. Converting a Single-Ended Signal with the AD7984 Differential PulSAR ADC <ul><li>Source: ANALOG DEVICES </li></ul>
  2. 2. Introduction <ul><li>Purpose </li></ul><ul><ul><li>This training module discusses the reference design converting a single-ended input signal to a differential signal for use with the AD7984 18-bit, PULSAR® ADC, and selected the ADA4941 ADC driver and the ADR435 ultra-low noise voltage reference. </li></ul></ul><ul><li>Outline </li></ul><ul><ul><li>Comparison between single-ended and differential inputs </li></ul></ul><ul><ul><li>PulSAR conversion technique </li></ul></ul><ul><ul><li>Overview of AD7984 ADC </li></ul></ul><ul><ul><li>Discussion of the verified circuits </li></ul></ul><ul><li>Content </li></ul><ul><ul><li>13 pages </li></ul></ul>
  3. 3. Single-Ended vs. Differential Inputs <ul><li>In single-ended inputs, one wire is connected from each signal source to the data acquisition interface. </li></ul><ul><ul><li>All inputs are referenced to a common ground. </li></ul></ul><ul><ul><li>Disadvantage: Single-ended inputs are sensitive to noise errors. </li></ul></ul><ul><li>With differential inputs, two signal wires run from each signal source. </li></ul><ul><ul><li>One goes to a + input and one to a - input. </li></ul></ul><ul><ul><li>Disadvantage: They need twice as many wires to single-ended inputs </li></ul></ul>
  4. 4. PulSAR ® Technique <ul><li>In the acquisition phase, the switches are closed to be connected to the IN+ and IN- analog inputs, thus each capacitor is used as a sampling capacitor acquiring the analog signal at the input. </li></ul><ul><li>In the conversion phase, the analog inputs are disconnected from the internal capacitors and applied to the comparator inputs. </li></ul><ul><li>Switching each element of the array between REF and REFGND starting with the MSB, brings the comparator back into a balanced condition and thus generates the output code representing the analog input signal. </li></ul>
  5. 5. Overview of the AD7984 <ul><li>18-bit resolution with no missing codes </li></ul><ul><li>Throughput: 1.33 MSPS </li></ul><ul><li>Low power dissipation: 10.5 mW at 1.33 MSPS </li></ul><ul><li>INL: ±2.25 LSB maximum </li></ul><ul><li>True differential analog input range: ±V REF 0 V to V REF with V REF between 2.9 V to 5.0 V </li></ul><ul><li>Dynamic range: 99.7 dB typical </li></ul><ul><li>No pipeline delay </li></ul><ul><li>Single-supply 2.5 V operation with 1.8 V/2.5 V/3 V/5 V logic interface </li></ul><ul><li>SPI-compatible serial interface </li></ul><ul><li>Ability to daisy-chain multiple ADCs and busy indicator </li></ul>
  6. 6. Analog Inputs of the AD7984 <ul><li>The analog input structure allows the sampling of the true differential signal between IN+ and IN−. </li></ul><ul><li>D1 and D2 provide ESD protection for the IN+ and IN- analog inputs. </li></ul><ul><ul><li>They can handle a forward-biased current of 130 mA maximum . </li></ul></ul><ul><li>The analog input signal can not exceed the reference input voltage (REF) by more than 0.3V. </li></ul><ul><ul><li>If the analog input signal exceeds this level, the diodes become forward-biased and start conducting current. </li></ul></ul>Equivalent Analog Input Circuit
  7. 7. Driving the AD7984 <ul><li>The noise generated by the driver amplifier must be kept as low as possible to preserve the SNR and transition noise performance of the AD7984. </li></ul><ul><ul><li>The typical noise of the AD7984 is 36.24μV rms </li></ul></ul><ul><ul><li>The SNR degradation due to the amplifier is </li></ul></ul><ul><li>For AC applications, the driver should have a THD performance commensurate with the AD7984. </li></ul><ul><li>For multi-channel multiplexed applications, the driver amplifier and the AD7984 analog input circuit must settle for a full-scale step onto the capacitor array at an 18-bit level (0.0004%, 4ppm). </li></ul>
  8. 8. ADA4941 Single-Ended to Differential Driver <ul><li>The voltage applied to the REF pin appears as the output common-mode voltage. </li></ul><ul><li>The voltage applied to the REF pin does not affect the voltage at the OUT+ pin. </li></ul><ul><li>Differential offset can exist between the outputs, while the desired output common-mode voltage is present. </li></ul>A2 A1
  9. 9. The ADA4941 Driving the AD7984 <ul><li>R1 and R2 set the attenuation ratio between the input range and the ADC range. </li></ul><ul><li>The ratio of R2 to R1 should be equal to the ratio of REF to the peak-to-peak input voltage. </li></ul><ul><li>R3 and R4 set the common mode on the IN− input. </li></ul><ul><li>R5 and R6 set the common mode on the IN+ input of the ADC. </li></ul>V IN
  10. 10. Verified Circuit Verified Circuits are designed for ease of use and proven to work by ADI.
  11. 11. Resistors Selections for the Verified Circuit <ul><li>The ADC’s common mode, which is equal to the voltage present at Voffset1, should be close to VREF/2. </li></ul><ul><li>The voltage present at Voffset2 should roughly be set to the ratio of Voffset1 to 1+R2/R1. </li></ul>10.00K Ω 20.0K Ω 1.00K Ω 2.00K Ω 5.00, 0.00 0.00, 5.00 1.667 2.5 ±5.5 10.00K Ω 15.0K Ω 1.00K Ω 4.02K Ω 4.99, 0.01 0.01, 4.99 2.000 2.5 ±10 10.00K Ω 12.70K Ω 1.00K Ω 8.06K Ω 5.01, 0.04 -0.01, 4.96 2.203 2.5 ±20 R3=R5=R6 R4 R2 R1 OUTN OUTP V offset2 V offset1 V IN
  12. 12. Power Supply for the AD7984 <ul><li>The digital input/output interface supply (VIO) allows direct interface with any logic between 1.8 V and 5.5 V. </li></ul><ul><li>To reduce the number of supplies needed, VIO and VDD can be tied together. </li></ul><ul><li>The AD7984 is independent of power supply sequencing between VIO and VDD. </li></ul><ul><li>To ensure optimum performance, VDD should be roughly half of REF, the voltage reference input. </li></ul><ul><ul><li>If REF is 5.0 V, VDD should be set to 2.5 V (±5%). </li></ul></ul>
  13. 13. Additional Resource <ul><li>For ordering the AD7984 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_CN0033/fca.html </li></ul></ul>

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