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TLV5620 reference questions

Mike Tripoli
Genius 3295 points
Other Parts Discussed in Thread: DAC7564, TLV5620, DAC084S085

I tried posting to the "etc" forum but got no response. I have a couple of questions regarding this device:

I have samples  and have tested it for operation at 5 volts. My question before was "will the device operate at 5V". So far, it appears that it does (the data sheet shows up to 5.25V). 

I have never seen a DAC where the reference voltage can't go to Vdd; the data sheet says "Vdd-1.5V". In my testing it doesn't matter what the reference voltage is set to; any code sent above 127 results in a full-scale output. I've tested the device at 3.3V with a 2V reference  (as shown on page 8 of the data sheet). I've tried 5V with a 3.5V ref. as well as 5V with a 5V ref. Regardless of Vdd-vref, 127 results in full-scale output. This is with the RNG bit set to zero (0). If the range bit is set to 1 then the DAC stops outputting data entirely.

I'm not locked into this DAC; I need one where all 4 DAC's can be loaded and updated simultaneously. This has the "LDAC" pin to achieve this. 

Any help is appreciated.

MT

  • 0
    TI__Mastermind 46290 points

    Hello Mike,

    I apologize for the delays. I do not regularly check the "etc." forum for Precision DAC posts.

    Mike Tripoli said:
    I have samples  and have tested it for operation at 5 volts. My question before was "will the device operate at 5V". So far, it appears that it does (the data sheet shows up to 5.25V). 

    This is fine.

    Mike Tripoli said:
    I have never seen a DAC where the reference voltage can't go to Vdd; the data sheet says "Vdd-1.5V". In my testing it doesn't matter what the reference voltage is set to; any code sent above 127 results in a full-scale output. I've tested the device at 3.3V with a 2V reference  (as shown on page 8 of the data sheet). I've tried 5V with a 3.5V ref. as well as 5V with a 5V ref. Regardless of Vdd-vref, 127 results in full-scale output. This is with the RNG bit set to zero (0). If the range bit is set to 1 then the DAC stops outputting data entirely.

    These parts are old, so finding documentation from their development now is pretty tricky. What I would imagine is going on here is that the operating conditions recommend operating with 1.5V of headroom on VDD to make sure that you keep the output amplifiers in a linear region of operation. You can probably get pretty close to the VREF at the output, but various specifications may deviate from how they are specified in the table (perhaps it takes longer to settle, etc.).

    With the range bit set to 1, you cannot achieve any functionality regardless of VREF? Or, is it just that when VREF is close to VDD and the range bit is set to 1 you cannot update the output? 

    Mike Tripoli said:
    I'm not locked into this DAC; I need one where all 4 DAC's can be loaded and updated simultaneously. This has the "LDAC" pin to achieve this.

    There are other ways to achieve this. Some device, such as the DAC7564, have an LDAC bit in the serial input word. Page 33 of its datasheet shows an example.

    If you can share more information about your requirements for the DAC, I would be happy to help you double-check our portfolio.

  • 0 in reply to Kevin Duke
    Genius 3295 points

    Thanks, Kevin.

    Regarding the TLV5620; this is a strange beast. As I said, the "codes" did not jive with what was coming out of the thing. I'm still not sure what that's all about.

    My first choice, and what I will use in my design, was the DAC084S085. The data sheet doesn't "spell it out", but it has a "load DAC" capability. One of the "operation bits, bit OP0 in particular, acts as a "load DAC" function. In the data sheet it says "Write to specified register and update outputs". I had to do some trial and error to see that this in fact writes all DAC outputs simultaneously. So, in my application I write to each DAC register using the OP code "00"; "Write to specified register but do not update outputs". On the "last pass" I set the OP0 bit to then update all outputs with the loaded data. The reason I'm "spelling this out" is that it's not obvious from the data sheet or the description that this part has this functionality. In scanning the parametric table for a DAC that does this (updates all outputs at the same time), if I had seen something like "Has LDAC" or "Can update all outputs simultaneously" I would have focused on this device earlier. Honestly, it wasn't until I saw an Analog Devices part that is virtually identical to this device (a couple of pins are swapped) that it occurred to me that this device could do this. The Analog Devices part codes exactly the same as this device, except they call the OP0 bit "LDAC". All in the marketing, I guess... 

    Mike Tripoli

  • 0 in reply to Mike Tripoli
    TI__Mastermind 46290 points

    Mike,

    Thanks for your feedback. We've been putting effort into improvements to our parametric search utility recently, I'll see what we can do to include "synchronous update" as a special features item.