LMH32404: Connection to sourcing photodiode and supplying negative VOD

Hi Sariel,

1. Can you attach the Pspice simulation file that you are running? I downloaded the reference TINA schematic from the LMH32404 product page and the model supports VOD adjustment. I will also work on setting up the simulation in Pspice to see if I observe the same thing.

2. There should not be a significant difference in the linear range of the device, regardless of whether it is in a sinking or sourcing current configuration. Sending the Pspice simulation file will help in troubleshooting why you are seeing a significant difference.

3. Section7.3.3 datasheet addresses how to use the VOD pin. As that section states, the VOD pin is only functional when the photodiode is sinking the photocurrent, not when the photodiode is sourcing the current. So unfortunately for your application, the VOD pin will not be functional and should be set to 0V.

Thanks,

Nick

Hi Nick,

Thank you for your response.

I have attached the simulator picture showcasing the LMH32404 performance. I utilized LTSPICE for simulating its behavior.

I observed that VOD (VOS control) does not impact the output signal, irrespective of its value and polarity.

Upon comparing simulation results with the LMH32401 under the same setup (configured for high gain), I noted a difference. The VOD control seems effective for the LMH32401 model, But for the LMH32401 model, the linear range is significantly lower when current is sourced into the device. The output saturates at a much lower current level compared to when I reverse the current source direction and sink current from the device.

Can I assume that I will achieve similar performance from the LMH32404 and LMH32401 when the LMH32401 is configured for high gain?

Considering the necessity of VOD=0 for current sourcing, what would you recommend for interfacing this device with an ADC that provides 2s complement data output?

Thanks,

Sariel

Hi Sariel,

Unfortunately, we do not have an integrated TIA solution that can create a offset of the differential output signal when you have a photodiode that is sourcing the photocurrent. For your application is it a requirement that the photodiode is sourcing the photocurrent or is it possible to configure the photodiode so that it is sinking the photocurrent? Also, are you planning on using all four channels or is your application one channel like shown in your simulation?

If it is not possible to have the photodiode configured to sink the photocurrent, then it is possible to still get the desired output signal. However, it requires a two-amplifier solution. If you connect a TIA amplifier and a fully differential amplifier that has the noninverting input biased, you will be able to get similar results. I can assist you further with that design if you can provide more details about the application requirements (i.e. input current range, bandwidth requirements, output voltage range).

Thanks,

Nick

Hello Nick,

My primary focus in the application is to ensure that the photodiode serves as the current source into the TIA.

I would need a compelling reason from the electronics' perspective to consider reversing the polarity of the photodiode.

My initial preference is to explore the utilization of the LMH32404, especially for its four-channel capability. The LMH32404 stands out as an excellent device, offering a compact size for four channels with outstanding performance.

If, for some reason, the LMH32404 isn't suitable, my alternative would involve constructing a four-channel solution using discrete TIA. Each TIA would be connected to an SE-DIFF FDA with the appropriate biasing. However, this approach would necessitate the use of eight devices instead of the single LMH32404.

Best, Sariel

Hi Sariel,

The LMH32404 is an excellent device that was optimized for easy design and connect of TIA applications with photodiodes configured to sink current. Therefore, the device is not well suited for an application like yours where you need to have an offset so there is never a zero differential voltage while you have the photodiode configured to source current. 

As you suggested for your alternative solution, constructing a discrete TIA connected to a SE-DIFF FDA is my recommendation for being able to offset the output differential voltage. I have constructed a design of the circuit using the OPA855 as the discrete TIA and the THS4541 as the differential amplifier.

OPA855_THS4541_TIA_E2E.TSC

If you have specific circuit or device requirements that you need to meet, I can further support adjusting the design to meet your applications needs.

Thanks,

Nick