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LMH6882: Gain in parallel mode

Luis M Brugarolas
Intellectual 295 points

Other Parts Discussed in Thread: LMH6882, LMH6881

I have been trying to find the LMH6882 gain in parallel mode and I am unable to find unambiguous relation between D0:D3 lines and gain. Checking AN-2267 application note makes things even more confusing to me, because actual measurements reveal information contained in AN is correct.

A table or a formula would be of great help.

Best regards

Luis Miguel

  • 0
    TI__Genius 13935 points

    Hello Luis,

    There is some ambiguity becasue there is only 20dB of gain adjustment range while numerically there is 31.75 dB of combinations, so many of the gain combinations are redundant.  In particular, once D0 and D1 are set the amplifier is in maximum attenuation and the other gain pins have no impcact.  See below for more details. 

    This is further complicated by the fact that D0 is pulled low on the eval board so that it can function as SDO in serial mode.  The DIP switch setting does not change the value of D0 on the eval board unless the board is reworked manually. The result of this is that the 0.25dB gain step is not available on the evaluation board. 

    6dB <= Gain <= 26dB

    Gain =  26dB - Attenuation 

    Attenuation =  (D6*16 + D5 *8 + D4*4 + D3 *2 + D2*1 + D1* 0.5 + D0*0.25)  AND  Attenuation <= 20dB

    Please let me know if you would like more clarification. 

    Thanks

    Loren

     

     

  • 0 in reply to Loren Siebert 1
    Intellectual 295 points

    Dear Loren,

    Thank you for your message. It adds very important information missing in the data sheet, as far as I know.

    To make it crystal clear I will rewrite your previous equation:

    Attenuation =  (RD6*16 + RD5 *8 + RD4*4 + RD3 *2 + RD2*1 + RD1* 0.5 + RD0*0.25)  AND  Attenuation <= 20dB

    Gain = 26 dB - Attenuation

    where RDx is Register Data.

    External pin data (PDx) are mapped in the following way:

    • PD0 -> RD3
    • PD1 -> RD4
    • PD2 -> RD5
    • PD3 -> RD6

    Thus, for pin data, previous equation may be rewriten to:

    Attenuation =  (PD3*16 + PD2 *8 + PD1*4 + PD0 *2)  AND  Attenuation <= 20dB


    In reference to evaluation board (AN-2267), there is a mistake in Table 2. It is disordered, there is a line missing and one line error:

    SW7/PD3 SW6/PD2 SW5/PD2 LMH6881 gain
    ON ON ON 26 dB
    ON ON OFF 22 dB
    ON OFF ON 18 dB
    ON OFF OFF 14 dB
    OFF ON ON 10 dB
    OFF ON OFF 6 dB

    Now, everything is clear and correct according to measurements. As St Thomas Aquinus said: "Veritas adequatio rex ed intellectum", truth is the correpondence between things -reality- and reasoning.

    Best regards!

    Luis Miguel

  • 0 in reply to Luis M Brugarolas
    Prodigy 20 points

    We got LMH6881 with evaluation board. We are trying to set amplifier to max gain of 26dB in parallel mode. We set all switches (D3 - D1 to logic zero). But we don't get correct gain. We get correct behavior until gain of 18dB but not for gain higher than that. We also noticed that total current drain is aroud 104mA until 18dB setting and jumps to 136mA for gain setting higher than 18dB. What could be the reason.

    Thanks for your help.

    Manish

  • 0 in reply to Manish Shah
    TI__Genius 13935 points

    Hello Manish,

    What is the common mode voltage setting?  What input signal amplitude are you using? 

    The jump in supply current seems to indicate that something is exceeding the normal operating ratings of the amplifier. 

  • 0 in reply to Loren Siebert 1
    Prodigy 20 points

    Hi Loren,

     

    I think we found an issue. In 26dB gain mode amplifier is becoming unstable. We were loading the amp with 200 ohm(differential)//7pf load (differential probe). It seems to be stable with 1pf load. Is that expected? What is the max capacitance can it drive without stability issue? We are terminating differential output with 200 ohm resistor. Should we do that or keep it open circuit and just load with high impedance differential probe?

     We are using CM of 2.5V and 5mv differential input. In our application we are measuring small signal (few mv) high bandwidth signal (up to 20Mhz). What is the maximum common mode voltage can it handle?

    Thanks,

    Manish

  • 0 in reply to Manish Shah
    TI__Genius 13935 points

    Hi Manish,

    Yes, this is expected behavior.  The maximum capacitive load is hard to specify because it depends on the complete output circuit, including PCB parasitic elements. 

    Open circuit is worst case for instability, so with no 200 Ohm termination the oscillation would be worse.  A 200 Ohm resistor is a good load condition.  What would work well for testing purposes is to break the 200 Ohms up into 3 resistors.  Use two 50 Ohm resistors on the amplifier output pins, then have one 100 Ohm resistor connected between the 50 Ohm resistors.  Use the 100 Ohm resistor as the measurement load (voltage will be 1/2 of the total output voltage).  By having the two 50 Ohm resistors between the measurement probe and the amplifier output pins the capacitance of the measurement probe will not cause instability. 

     The range for the output common mode is 2.0 to 3.0V.  2.5V is the optimum voltage. 

    Regards,

    Loren