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# LMH6521 Output Gain

Other Parts Discussed in Thread: LMH6521

Hallo,

I have a question.
I think that the output gain of the LMH6521 is (26dB – Attenuator Gain).
Is my understanding correct?

They set the digital attenuator to 18dB (C8h) referring to Table 2 in the datasheet, using Serial interface.
But, they said the output gain had been 1/2(minus 6dB) of the calculated gain.
Their setting are;
INA+/INA- inputs = 2Vp-p sine wave and AC coupling
Digital attenuator = 18dB   (Expected output gain = 26dB – 18dB = 8dB)
They said OUTA+/OUTA- had output 2.5Vp-p, though they expected 5Vp-p output.
Is there any other factor of additional x1/2(minus 6dB) gain?

Best Regards,
Kazu Ogawa

• Hi Kazu,

Is the customer evaluating the LMH6521 using an evaluation board?

The LMH6521 has 10ohms internal series resistors and the evaluation board has 40ohms series resistors on each output to the SMA connector effectively providing a differential 100ohms series resitor to each SMA output. This when driven at the output to an 100ohm termination/load would provide a voltage division of 100/(100 +100) = 1/2 or 6dB loss. I think this might be a reason as to why the customer is seeing 6dB reduced gain.

The 40ohms series resistors are there to provide stability at the outputs by isolating the outputs from large parasitics. It might be possible to reduce the series resistor values to 10ohms on each output and design an un-balanced filter in-between the LMH6521 and the ADC, if customer desires lesser loss at the outputs due to the series resistors.

Please let us know if the issue is resolved or not.

Thanks & Best Regards,

Rohit

• Rohit-san,

I will give this advice to the customer.
If there is any application note or material regarding this phenomenon, would you please give me it?
Because it’s difficult to know this phenomenon from only the LMH6521 datasheet.

Best Regards,
Kazu Ogawa

• Hi Kazu-san,

I have attached a link to the LMH6521 evaluation board which should describe the circuitry on the evaluation board thoroughly: http://www.ti.com/tool/lmh6521eval. Please let us know if the issue is resolved.

Thanks & Best Regards,

Rohit

• Rohit-san,

The customer asked me how to calculate the output voltage.
I think, for Figure 27. Basic Operating Connection in the LMH6521 datasheet 12-page,
Voltage between OUT+ and OUT- = 5V(p-p)
Voltage between both ends of 100-ohm = 5V x (100/(10+40.2+100+40.2+10)) = 2.5V
Is this correct?
Also, is the same manner correct in the LMH6521 EVM, though the EVM has the transformer connected OUT+/OUT-?

Best Regards,
Kazu Ogawa

• Kazu-san,

Yes. The Voltage between the OUT+ and OUT- pins will be 5V(p-p), and voltage between the ends of 100ohms will be 2.5V(p-p). The voltage division you have mentioned ofcourse depends upon the series resistor at the output pins which in your case would be 40.2 ohms on each output.

It should be the same manner for the EVM with the transformer connected at OUT+/OUT- SMA connectors.

Best Regards,

Rohit

• Rohit-san,

Thank you very much.

Best Regards,

Kazu Ogawa