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DAC8820: Suggestions required for a low noise, fast settling DAC for pretuning VCO in a PLL

Part Number: DAC8820
Other Parts Discussed in Thread: TLC7524, , OPA2277, THS4031


I need some suggestions to select a fast settling DAC part which I want to use to pretune a VCO inside of a PLL loop for faster frequency switching time (< 1 us). I want to combine the PLL loop filter output and the DAC output in a non-inverting summing circuit which will output the Vtune for the VCO. In this regard I want to know

1. How the "Output spot noise voltage" specification of the DAC might influence the overall phase noise of the PLL? Is 10 nV/sqrt. Hz as in DAC8820 a low enough specs for this application? What is this spec for TLC7524 which has a faster 100ns settling time?

2. Is it preferred to switch the pretune DAC output to the loop filter output using a SPDT once the DAC is settled or combine them both in a summing circuit?

3. Will a high slew rate OPAMP e.g. THS4031 be able to meet the settling time specs. of DAC8820 instead of using OPA2277?

4. Any other TI DAC suitable for this application preferrably with a parallel interface? please suggest.


  • Hello, 

    The DAC8820 and TLC7524 parallel MDACs are going to be the fastest settling. The settling time will be dominated by whichever device (DAC or op amp) has a higher settling time. To meet the settling time spec in the datasheet you'd need to find an op amp that has a lower settling time than the DAC. The THS4031 looks to be the faster of the two op amps. We do not have the noise spec for the TLC7524.


    Katlynne Jones