LMH6401 with single ended input

Hi Noam,

I would not recommend using the LMH6401 with single ended input, as it is designed to be driven differentially and its common mode rejection would be poorer if driven single-ended. I would recommend preceding the LMH6401 with an FDA like the LMH3401 or LMH5401, if you want to do single-ended to differential conversion. In this way, you can use the DC DAC on the un-used FDA input to cancel out the DC offset.

Best Regards,

Rohit

Hi Rohit,

Thanks for your reply.

I have a very minimal area for the TIA + VGA, I can't add another component in-between.

I don't understand how the CMRR is relevant here. I'm implementing a single-ended input so the CMRR is irrelevant (SE input CMRR is actually 0dB...)

I've attached TINA simulation.

Thanks,

Noam 

LMH6629 fronend1.TSC

Hello Noam,

Do you require good linearity in this circuit? 

When one input of a differential amplifier is grounded there is a common mode input to the amplifier.  For Vinm grounded (Vinm= C; constant voltage ), the differential input is Vin, diff = (Vinp - C), however the common mode input is Vin, cm = (Vinp + C)/2.  Hence the common mode signal into the amplifier is only 6dB down from the differential input signal.  {edit: corrected cm equation above}

Many of our FDAs have an output common mode loop that cancels out this input common mode signal.  The LMH6401 does not have a common mode cancellation loop that is sufficient to drive the amplifier with a single ended signal.  So, there is a significant feed through of common mode signal to the amplifier output, hence Rohit commenting on the CMRR.  Also, the amplifier is not very linear with operated with a single ended input.

It may be possible to replace the TIA with an FDA in a TIA configuration.  The LMH6554 or LMH5401 would be most suitable in TIA configurations.  The LMH3401 has on chip resistors and is intended as a voltage amplifier.

I looked at your TINA circuit.  The Vocm pin should not be connected to the amplifier outputs. DAC3 should be the only connection to the VCM pin.  DAC3 should not be connected to the output pins, any common mode offset will create extra current through R7 and R8.  I did run transient simulations and the circuit does work.  It is possible that if you do not need high linearity (pulse detection) this circuit would work for you. 

One possible alternative is to use an LMH6881 programmable gain amplifier instead of an LMH6401.  These amplifiers were designed for single ended inputs. 

Regards,

Loren

Thanks Loren for the answer,

I'll consider the LMH6881 instead.

Thanks

Noam

Hello,

I am using LMH 6401 in ac coupled configuration to amplify the AC signal which is then followed by a RF power detector to convert the same signal into DC. I am a little bit confused with PD and Vocm Pins since i pulled PD pin to low (connected to ground) and Vocm is also pulled to ground with 10nF capacitor. Is this right or did i make some mistake for these pins.

Regarding gain amplification which ca be done digitally , can i get some help in that regards.

Thanks and Regards

Note : i posted the same on the forum but did not get any reply so if you can help with my question.

Hi Raza,

If you have connected the PD pin to ground, then the LMH6401 part is likely powered on. I think it would be better if you could attach the LMH6401 circuit diagram in-order for me to answer your questions more accurately. 

The gain amplification can be done by writing to register 2 as shown in the SPI Register map. Writing a value of 0x00 Hex to register 2 will set the LMH6401 to the maximum gain setting of 26dB.  

Best Regards,

Rohit

Dear Rohit,

Can you help me with the code also for maximum gain .

Thanks

Hi Raza,

Could you let us know in detail what help would you need to code for the maximum gain? The gain control field description is provided in Table 6 of the datasheet which I have attached below. Just make sure you don't program a 1 on the 6th bit of the gain control register, or else you will power down the part. You could also consult table 10 in the datasheet that provides the value in hexadecimal format that you need to write into address 0x02 hex for different gain configurations.

Best Regards,

Rohit

Dear Rohit,

I am not too good in programming and  the amplifier need to be controlled digitally, as far as i need to put the amplifier all the time for the maximum gain of 26dB to amplify my ac signal so if you can provide me the code for this.

Thanks and Regards

Hi Raza,

Please take a look at the LMH6401 EVM GUI User's guide: www.ti.com/.../lmh6401evm for instructions on how to program the LMH6401 device. The LMH6401EVM GUI software should also be available in the same link.

Best Regards,
Rohit

Dear Rohit,

Please note that i am not using evaluation module for LMH6401 , i am using it on my PCB which will be controlled from micro controller.

Regards

Hi Raza,

I think the programming needed to set the LMH6401 at maximum gain is very basic, where you need to write a 0x00 hex (or 00000000 binary) number to the address 0x02 hex (00000010 binary).

You might want to take a look at Figure 47 on Page 21 of the datasheet that shows the SPI write cycle. In the figure 47, the A6 to A0 are address bits and the D7 to D0 are data bits. For changing the LMH6401 gain, you need to have 0000010 binary number as the address bits (A6 to A0). The D7 to D0 data bits will be 00000000 binary number for maximum gain.

Best Regards,

Rohit