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ADS5400: When use LMH5401 as DC-Couple interface to ADS5400, the ADC output is over range

xiang song
Intellectual 770 points
Part Number: ADS5400
Other Parts Discussed in Thread: LMH5401

hi all:

 

My hardware is as follow:

1)The ADS5400 is internal reference;

2)LMH5401 is powered by single power supply: VS+ is 5V and VS- is GND;

3)The output of LMH5401 is followed by a low-pass filter as disribed in Figure 37 of ADS5400's datesheet;

4)without input the output of ADS5400 is all 12'h800;

5)Now when I input a sin signal by a signal generator, the output of ADS5400 is as follow:

The simulation result is as follow:

Why? 

=============================================================================

Now, I add a resister as follow:

The output of ADS5400 is ok for input signal power below -10dBm,  But without input signal, the output of ADS5400 is so big.(signed Decimal almost 980)

Thanks!

  • 0
    TI__Genius 13935 points

    Hello Song,

    Have you simulated the circuit with the added resistor? 

    Your ADC reverence voltage is 2.5V.  Your signal reference voltage is 0V.  This creates a potential for DC offset voltages to be developed.

    These offset voltages are presented to the ADC and will be digitized.  Adding resistor R264 changes the DC offset voltage at the amplifier input.

    You will see these offsets more clearly if you plot both outputs during simulation.  You are looking at Vout_p, so you only need to add Vout_n to see these offsets.

    Regards,

    Loren