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DAC8820: Output voltage of DAC8820

Part Number: DAC8820

Hello,

I am considering analog output circuit that circuit is used DAC8820.
And I am considering this circuit by referring to the contents of Fig. 42 of the DAC8820 data sheet.
I want to make sure that the output voltage of the circuit in Figure 42 is the voltage shown below.
I'm new to multiplying DACs, so let me check their behavior just in case.

Case 1: VREF=+10V DAC data=65535 VOUT=+10V

Case 2: VREF=+10V DAC data=0 VOUT=-10V

Case 3: VREF=-10V DAC data=65535 VOUT=-10V

Case 4: VREF=-10V DAC data=0 VOUT=+10V

* AC voltage (DC to 8MHz +/-15V) can be input to VREF.

Regards,

MESH

  • Hi Mesh,

    You have a correct understanding of the bipolar circuit.  The transfer function is correct as you have described it.

    One thing to consider is that bandwidth is dependent on the resistance of the resistor ladder, which is dependent on the DAC code.  For that reason, you do not have 8MHz bandwidth for all codes. 

    Thanks,

    Paul

  • Hello, Paul

    Thank you for your reply.

    I understand multiplying behavior of DAC8820.

    However, I don't understand the meaning of Figure 9 and Figure 10 in the the DAC8820 data sheet, so please tell me.

    I considering reference input of sine wave of 100Hz to 200Hz +/-10V.

    Regards,

    MESH

  • Hi MESH,

    Consider that the MDAC has code-dependent output impedance between the reference and the IOUT pin.

    This resistance creates an RC low pass filter response with the parasitic capacitance on the output.  We specify the output capacitance as ~50pF.

    In the case of the bipolar circuit, the output impedance as seen by the output amplifier is a bit complex, but basically, you will see a filter response on the gain graph that has a cutoff of -20dB per decade.  As your input frequency is pretty low, you should not see much bandwidth decrease for most codes.

    Thanks,

    Paul

  • Hello. Paul

    Thank you for your reply.
    I read your answer.
    I rechecked the characteristic curves in Figures 9 and 10 on the DAC8820 data sheet.
    Should I pay attention to the fact that the frequency response is reduced in the band exceeding 1MHz in the characteristic curve of the data sheet?
    (The reason why the frequency response reduces is the ladder circuit and the capacitance of 50pF)

    Regards,

    MESH

  • Hi MESH,

    The bandwidth reduces when frequency is >1MHz due to the output amplifier bandwidth limits.  There can also be some inductive properties of the DAC that can effect the high frequency bandwidth.

    Thanks,

    Paul

  • Hello, Paul

    Thank you for your reoly.

    Please let me confirm which part of the influence by ladder resistance and 50pF should be focused on in the graphs in Figures 9 and 10.

    Regards,

    MESH

  • Consider that the IOUT pin looks like variable resistor or potentiometer.  The parasitic cap on the output creates an RC filter.  The resistance is dependent on the DAC code.  That is why bandwidth is limited by the DAC code.

  • Hello, Paul

    Thank you for your reply.

    I understand.

    Regards,

    MESH