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THS4520 output range with Single ended supply

Xiaochen (Sean)Xu
Expert 6040 points
Other Parts Discussed in Thread: THS4520, THS4130, THS770012

Hi, 

we are using the TSH4520 to generate a differential output with CM=0.75V. 

the differential output range is from 0 to 1.5Vpp. That means the THS4520 differential outputs will be between 0V and 1.5V respectively, consdiering the CM=0.75V.

The supply for 4520 is a single ended supply 0/5V.  usually the opamp needs to have some margin between the lowest voltage output  and negative supply .

in our case,  when THS4520 output is 0V, can  the 0V output be achieved without perforamnce degradation? if not, what performance specs can be affected when we use 0/5V supply and OUTM is around 0V,e.g. noise, harmonic distortion or etc?

does dual supply have to be used for our applicaiton? we have +/-5V supply on board, but 4520 doesn't support over +/-3V.  Can the THS4130 be the next candidate  if we have  to use +/-5V?

Thanks!

xiaochen

  • 0
    TI__Expert 4115 points

    xiaochen,

    The THS4520 data sheet specifies each output can swing +/-1.4V (typ) with +/-1.65V supply and 100 ohm load to mid-supply (see page 7).  In other words to within 250mV from each supply voltage. This will improve by a 100mV or more with lighter loading like 1k to 10k.

    If what you explain is correct, this would mean it will not work. But differential voltage swing is double the single ended swing, and makes it closer. Figure 46 and Figure 47 on page 17 of the data sheet shows HD2 and HD3 with 2Vpp and 200 ohm load. It takes some work, but interpolation your voltages to this graph, the HD performance should be similar to Vocm = -0.625. This is in the region where preformance is getting bad. However if the load is not so heavy (like 1k to 10k) it may work.

    Another appoach to the problem is to set the amplifier Vocm = 1.5V (mid-supply) and then use a resistor divider to lower the CM to the load. The technique is discussed on page 22 of the THS770012 data sheet. The table below show one scenario and Figure 46 and Figure 47 in the THS4520 data sheet shows HD performance at 3Vpp is OK.

    Component values for resistor divider to lower CM
    Vocm Vadc Vref Ri Ro Rp Gain Gain dB Vadc fs Vamp FS Vamp -1dB Vamp -6dB
    1.65 0.75 0 1000 50 41.7 0.44 -7.04 1.5 3.38 3.01 1.69

     Otherwise, the THS4130 with +/-5V suppy is good option. At lower frequencies it is extremely good HD and very low noise.

  • 0 in reply to Jim Karki
    TI__Expert 6040 points

    Jim,

     

    thanks for the detail reply!

    our load is >100Kohm , however but we still want to design it right and let the chip operate appororiately. i think we have multiple soluitons based on your reply. 

    Xiaochen