AD5761/21/R FAQ

Document created by msamera Employee on Mar 16, 2015Last modified by mbradley on Oct 5, 2015
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Q1. What external devices does the part require to operate as spec’d in the datasheet?

A1. The AD5761R/AD5721R are 16-/12-bit Voltage output DAC which integrate a 2.5V 2ppm/˚C typical internal reference as well as reference and output buffers. The device is able to perform as expected in the datasheet with the only addition of external power supply. The AD5761R/AD5721R will require unipolar or bipolar supply depending on the output voltage range configured and expected on the output.

 

The AD5761/AD5721, on the other hand, does not include an internal reference for which case an additional 2.5V external reference is required. After power-up ensure that the software reset as outlined in the data sheet is completed.

 

Q2. Is there a need to include external discrete devices to obtain any of the eight different output voltage ranges listed in the datasheet?

A2. One of the main features that this DAC offers is the flexibility and ease of use with regards to the output voltage ranges available within the DAC. The DAC prevents the user from spending time researching the correct op-amps and resistors with matching temperature coefficients and the hassle of not compromising board area. The eight different output ranges (+5V, +10V, +16V, +20V, ±3V, ±5V, ±10V, -2.5V to +7.5V) are programmable with a single write to the control register and the whole solution takes up 9mm2.

 

Q3. In order to modify the output voltage range, how is this done?

A3. Two simple steps are required when the output voltage range is reconfigured. A Software Full Reset of the part and a write to the Control Register to configure the device, remove the initial clamp to ground and power up the output buffer.

 

Q4. What is the difference between a Software Full Reset and a Software Data Reset?

A4. A Software Full Reset, like a hardware reset, brings the part to its power-up state. In this case the output is clamped to ground through a 1kΩ resistor and a write to the Control register is required to configure the DAC.

 

The Software Data Reset resets the output voltage to the power up voltage previously configured by the user. There are three possible reset values available within the DAC; Zero Scale, Midscale and Full Scale are selected on the PV [1:0] field in the Control Register (bits DB3 and DB4).

 

Q5. Initially I configured the part to operate in Standalone mode. How can I re-enable the daisy chain?

A5. The only way to enable the daisy chain mode for this part is to go through a power up cycle.

 

Q6. The AD5761R integrates an internal reference. How can I configure the part to work with it? Does the part require extra circuitry? If I decide to use an external reference, do I need to reset/reconfigure the part?

A6. The internal reference provided within the AD5761R is on at power up and no extra circuitry is needed when the reference is used as an input. If an external reference is preferred, the internal reference can be disabled during the initial write to the control register. The 2.5V 2ppm/˚C typical internal reference can also be used to drive external loads, in which case it is recommended to place a capacitor in the range of 1nF to 100nF to improve the noise performance.

 

Q7. What does TUE stand for? Why is it included in the datasheet?

A7. TUE stands for Total Unadjusted Error; it is effectively the worst case (measured in voltage) the actual output voltage can be from the ideal at any given code. These sources of error are offset error, gain error and linearity errors.

 

The AD5761R offers a TUE spec of 0.1%FSR with an external reference, while this error is only 0.15 %FSR when the DAC is operated with the internal reference.

 

Q8. Can the device output bipolar ranges from single supply? Is it possible to supply the DAC in a way that all output ranges are covered?

A8. The AD5761R requires dual supplies to output bipolar output ranges. There is a need to account for a maximum headroom of 1V on top of the output voltage range when deciding on the voltage supplies to apply to the DAC.

 

The AD5761R can be supplied with +21V positive analog supply (Vdd) and -11V negative analog supply (Vss) in order to have the capability to output any of the eight different programmable output ranges available. The 32V, result of the sum of the analog supplies, is within the Absolute Maximum Ratings of (-0.3V to +34V) provided in the datasheet. A digital supply in the range of 1.7V to 5.5V is also required.

 

Q9. The AD5761R offers an ALERT pin that is asserted low in case of die temperature over 150˚C, output short circuit and brownout. What does the brownout condition cover?

A9. DB11 in the Control Register, described as BO, is set to ‘1’ to inform of a brownout condition in the following situations:

  • Device at power up.
  • After a hardware reset. The part is set as on power up status.
  • After a Software Full Reset. The part is set as on power up status.
  • Positive analog supply reduced under its minimum specification. The part is set as on power up status.
  • Short on the DNC pin (pin 9 in TSSOP package; pin 7 in LFCSP package)
  • Working with the Thermal Shutdown alert enabled, the die temperature exceeds 150˚C. The internal digital supply powers down once the die temperature rises, so the part is set as on power up status.

 

Q10. In the case of an asynchronous Clear function, does the DAC output update immediately?

A10. Whenever there is high to low transition on the Clear signal, the DAC output is immediately updated to Zero Scale/Midscale/Full Scale based on the value of the CV[1:0] bits (DB9 and DB10) from the control register.

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