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THS4508 AC-coupled 50-Ohm SE to 100-Ohm Diff Problem

Victor Levandovsky
Intellectual 575 points
Other Parts Discussed in Thread: THS4508, TINA-TI, THS4520

Hello,

I`m designing 20 dB amplifier for 50-Ohm SE to 100-Ohm Diff. conversion.

I use circuit schematic from THS4508 Datasheet (Figure 53, page 21) with components value from Table 4.

http://www.ti.com/lit/ds/symlink/ths4508.pdf

Circuit behavior correct at 3.75 V (but not 20 dB) and absolutelly not correct with 5V. See TINA-TI design, please.

I think that problem with not correct Rpd resistor value, because Rpd increases with Gain for 6-10-14 dB

but have decreased value for 20 dB (table 4, page 21)

I think Equation3 is not correct because returns negative value.

You can check this using or Table 3.

You can see that all resistors the same in Tables 3 and 4 (only Rpd different)

Also equation for Rpd in AC-coupled mode is not provided.

== Regards, Victor

P.S.

My TINA-TI design in attachment

0724.DIFF_AMP_20dB_NEW.TSC

  • 0
    TI__Genius 13935 points

    Hello Victor,

    Page 20 of the datasheet says the the Rpd is only required when the supply voltage is less than 5V.  So, for 5V operation, remove the Rpd. 

    Rpd is not dependent on gain, but rather on the input common mode voltage you are trying to set. 

    Use the component values from Table 1 if you want to use a 5V supply. 

    Thanks for including the TINA file, that saves me a lot of time. 

    Regards,

    Loren

  • 0 in reply to Loren Siebert 1
    Intellectual 575 points

    Hi, Loren

    Thank you for information.

    I would like to notice two things:

    1) Fig 53 named as:

    THS4508 AC-Coupled Single-SourceSupply Range From 3.75 V to 5 V With Rpd Used
    To Set Vic.

    2) See attachment, please. No 20 dB with resistors value from Table 4  and 3.75 V.

    No amplification when 5V power used.

    I think problem with not correct Rpd calculation in table.

    -- Regards, Victor

    6457.DIFF_AMP_20.TSC

  • 0 in reply to Victor Levandovsky
    TI__Genius 13935 points

    Hello Victor,

     The datasheet specifies an input common mode range of 2.3V to -0.3V.  When I run a DC node analysis in TINA with the schematic that does not work I get an input common mode of 575mV which should be OK (but I think this is lower than optimal), but the output pins are at 3.9V. 

    The outputs are supposed to default to mid supply.  Even when I connect a power supply to the vcm pin the outputs are not at the correct voltage. 

    I do not t think that the resistor values are the problem because I tried changing them and the results were not as expected.  The resistor values from the table should work better than the spice simulation shows.

    The TINA circuit is not behaving like the datasheet specifications indicate.  The DC operating points of the output pins is not correct, this is thowing off the operation of the rest of the circuit.  I looked at the symbol and the SPICE model and it looks like everythig is connected property.  I am not sure what is wrong. 

    One option would be to try the same circuit with the THS4520 model.  That amplifier also has an input common mode rage that goes below 0V and the pull down resistors would work the same. 

     

    Loren

  • 0 in reply to Loren Siebert 1
    TI__Genius 13935 points

    Hello Victor,

    The TINA model is showing strange behavior.  For some reason using a Vocm of 2.5V on a 5V supply is showing bad results. If you force the Vcm pin to other voltages in the valid range then the circuit functions better. 

    You are also correct, the 348 Ohm Rf and a 40.2Ohm Rg does not give 20dB of gain. 

    Try the attached schematic.

    Rf7444.0724.DIFF_AMP_20dB_NEW.TSC

    I used Rf =402, Rg =23.7. 

    Loren