LMH32404: Filter and Balun Problems with LMH32404

Hello Ahmet,

Thank you for your question. 

1. Could you clarify which part of the device you are referring to with the "amplifier's filter"? This amplifier does not have an integrated filter, however it does have an integrated transimpedance gain of 40kΩ. 
   
   1. If you are looking for an even smaller size however to create more space, we do offer the LMH32404 and single-channel variant LMH32401 in Bare Die. This forgoes the plastic packaging all together, and is delivered on an easy to use tape and reel. 
2. Depending on the device chosen, a 1:1 transformer could be used to convert from a differential to single-ended signal. Additionally, a simple difference amplifier could be used. 
3. The LMH32404 has a multiplexing output so you could use 1 balun after the amplifier and switch between the output channels using pins M1-4. 

I also wanted to note that depending on your application, you may consider the LMH34400. This device has a single ended output which would eliminate the need for a balun on the input or output, and is in ultra-small SOT-5X3 package (1.6mm × 1.2mm). The LMH34400 also features a power-down pin that allows them to be used in a multiplexing configuration if multiple devices need to be tied to the input of the next stage of the signal chain. 

Thank you,
Rachel

Hello Rachel,

First of all, thank you for your reply.

For the filter question:

- Since LMH32404 has 6 VDD pins and each of them requires 33nF and 100pF capacitors (according to  LMH32404 EVM User's Guide documentation) the IC has 10 capacitors for it's power pins. Since this device is going to be as small as possible is there any way to reduce it?

Thank you for the rest of the advices and recommendations. I will check them if they can be a solution for my problem .

Thank you,

Ahmet

Hello Ahmet,

Thank you for the additional clarification. I am working with the design team to better understand what options could be available. 

Please allow me a little time to investigate this situation and provide any updates on the alternative recommendations made. 

Thank you,
Rachel

Hello Ahmet,

I spoke with the team on potential options. I have outlined a few details below to help you make a determination on what is best for your application. In general, our recommendation is to have the capacitors as outlined in the EVM however hopefully this information helps you make a determination for your design. 

• The smaller capacitors are typically present to reduce high frequency noise. If this is not a high priority, then that can be evaluated. 
• The larger capacitors are to help with big current surges. Please evaluate your power supply on what the need is here.
• Each input separate capacitor pairs to help reduce electrical crosstalk. Depending on your design this might not be as high as an issue, such as if you are multiplexing the output and only evaluating one output at a time, then you can consider sharing capacitors. 

Additionally, I mentioned previously that you might consider purchasing the LMH32404YR which is our bare die option. Bare Die devices allow you to create a MCM with wire bondable capacitors inside which would reduce the size of the design as well as see better performance. 

Thank you,
Rachel 

Hello Rachel,

After your recommendations I came up with a design. In this design there are 6 amplifiers (4 on first LMH32404 and 2 on second LMH32404). The working principle can be explained as: 

-Always one amplifier pair will work at a time (e.g 1st and 2nd amplifier or 3rd and 4th amplifier or 5th and 6th amplifier).

-Odd or even numbered amplifiers will never work on the same time (e.g 1st and 3rd amplifier will never work on the same time).

Considering your balun recommendation, I grouped amplifier outputs that will never work same time (1-3-5 and 2-4-6) and connected odd numbers to one balun and even numbers to another balun and control which pair is going to work with Multiplexing pins. Is this OK?

Since the MCU that I am going to use has limited GPIO pins since I will connect 3 pins already because of the multiplex operation, I can't connect all of the EN, IDC_EN, VOD and VOCM pins, thus I need to get rid of these pins as much as I can. I connected EN and IDC_EN pins to GND since they are active low but I have no idea what to do with VOD and VOCM pins. What are your recommendations for these 2 pins? What happens if I connect them to high or low? Or are they must to be tied to a controller?

I will use only 2 channels on the second amplifier. Can it's input capacitors reduced since i will not going to use other to channels? If yes, how?

I am adding my design below for your examination. Thank you for your answers.

Thank you,

Ahmet.

Hello Ahmet,

Glad to hear more updates and additional information. What type of wearable project are you working on? The application would have some affect into the granularity of recommendations I can make. In the meantime, below is some additional information to address your questions. Please let me know if there are any other questions. 

Ahmet Talha Güleryüz said:

Considering your balun recommendation, I grouped amplifier outputs that will never work same time (1-3-5 and 2-4-6) and connected odd numbers to one balun and even numbers to another balun and control which pair is going to work with Multiplexing pins. Is this OK?

This should be fine. Take care to ensure that there is enough time for the signal to switch entirely from one signal to another before reading the output on the balun. The LMH32404 has a 10ns channel switching time, but I cannot speak to the other components within the system. More information about using the 4-channel multiplexing functionality of the device can be found in our application brief, linked here.

Ahmet Talha Güleryüz said:

I connected EN and IDC_EN pins to GND since they are active low but I have no idea what to do with VOD and VOCM pins. What are your recommendations for these 2 pins? What happens if I connect them to high or low? Or are they must to be tied to a controller?

If you will not be using VOCM, it is recommended to place a bypass capacitor to ground on the VOCM pin. The amplifier will then self-bias the output common mode to approximately 1.1V. 

VOD is functional only when the LMH32404 is used with a photodiode that is sinking the photocurrent. You can use Equation 1 as outlined in section 7.3.3 of the LMH32404 datasheet to determine the value to set VOD to. If your photodiode is sourcing the current, set VOD = 0V. 

Section 7.3.3 Differential Output Stage of the LMH32404 Datasheet has more details about the functionality and use cases of these pins. 

Ahmet Talha Güleryüz said:

I will use only 2 channels on the second amplifier. Can it's input capacitors reduced since i will not going to use other to channels? If yes, how?

If you will not be using channels 3 and 4, you will not need to add those pairs of input capacitors. I would however recommend leaving 2 sets of input capacitors (one for each channel being used) because you will be looking at both outputs on that device at the same time. 

Thank you,
Rachel