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AD5422 Design queries

Question asked by Suresha on Jan 23, 2014
Latest reply on Jan 23, 2014 by Suresha

Hi all,

 

We are designing a new system with AD5422 . Our requirement is to support 4-20mA current output & +/- 10V voltage output.

We are supplying +13V & -13V to AVdd & Avss respectively. We have some queries regarding the design.

 

  • We want to maintain a separate ground for analog output signals. This is to avoid digital ground noise to effect analog output. Thus we have connected Avdd, Avss &  the output section with respect to analog ground. We have a doubt on whether to refer Reference input ( REFIN ) to analog ground or digital ground. Since absolute maximum rating of the datasheet suggests REFIN is referring to GND , we have referred REFIN pin to digital ground in the design . Is this correct ? Please comment.

 

  • Circuit Note CN0278, shows having a common load of 500Ω  for both voltage & current output. But the datasheet says minimum voltage output impedance is 1KΩ . How this is supported in CN0278 ? In our board maximum current output impedance is 500Ω & Minimum output voltage impedance is 200KΩ . Thus there will be change in the output impedance as soon as the output type varies from voltage to current or vice versa. Is this switch of output impedance is supported by the DAC ?

 

  • The datasheet also suggests using Boost function when operating at the extremes of the supply voltage, load current, and temperature range. We are operating with +/-13V supply, 4-20mA current & operating temperature range of -20˚C to 70˚C. Is external transistor required for this use case ?  In Boost mode operation , can we short the emitter of external transistor to the Vout pin ?

 

  • Regarding TVS protection, we have put a bidirectional TVS diode with a reverse standoff voltage of 10V & maximum breakdown voltage of 12.3V. Since 12.3V is less than Avdd & Avss, external clamping diodes will not be forward biased during a transient event. In this case can we remove the external clamping diodes ?

 

Looking forward to your responses.

 

Thanks & Regards,

Suresha N S


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