LMH32401: transimpedance amplifier

Hi XY,

section 7.3.4 of datasheet in combination with figure 7.2 answers your questions.

Kai

If unused, those pins can simply  be bypassed with external capacitors (optional) to reduce noise from the internal resistor divider networks. Vocm will defalut to 1.1V in this case. When setting the VOD pin it is important to consider whether the input is sourcing or sinking current.

Best,

Sam

Hi Sam

by datasheet implementation, if not used these two pin, should be decoupled it by a 0.1uF cap to avoid noise. but just a decouple cap, if measured the pin voltage, the Vod is ~1.1V, the Vocm is ~0.5V. I want to set these two pin to ~0V to match next fully amplifier like LMH5401 or LMH6401. The PD connection as datasheet, negative voltage to anode, cathode node connect to LMH32401 input pin. 

another way, the datasheet test condition is Vod=0V, how to set it? just decoupled it with cap as evaluation board. get confused. 

would you like to explain more about it?

Hi Xy,

For a single supply configuration you do not want the output common mode to be zero volts. As you can see below, the output common mode can be thought of as a middle point between the differential output swings. If you set Vocm to 0V you will not be able to output a differential signal, because that would require the output to swing negative. The VOD pin is used to balance the output to allow for higher dynamic range, If you tie it to 0V it is fine, but it may not be the best choice depending on the system.

One way to deal with the input common mode of the next stage is to AC couple the signal with series capacitors. I am curious what the supplies are for your next stage, do you have a mix of single and dual supplies in the system? Do you need the signal to be DC coupled?

Best,

Sam

Hi Sam

thanks for you update soon.

and as you mentioned plus the real situation, the input pulse current is very low, about hundred nA(such as 100nA). so if configure the Vocm to 0.5V(in the real application, the output level is low even high gain mode of lmh32401).  I tied the Vod pin to a cap to get the better bypass to avoid noise into the system.(the Vod default configuration is ~0.5V by the part block diagram). and so on, to try to meet the part specification. 

the next stage amplifier could accept 0.5V commode voltage even configure it to Vocm to 0V, such as LMH5401, not degrade the amplifier's performance obviously. the lm32401 output is dc coupled with next amplifier and lmh5401 used dull supplies(+/-2.5V)

so another question will be followed, I tried to simple the circuit design and cost down. 

1. could I use external resistor to tie to Vocm/Vod pin to get the desired voltage and plus a another decoupled cap to avoid noise like datasheet recommendation?

2. if LMH32401 used dull supplies, such as +/-1.65V( or +2V, -1.3V). as this configuration, could I set the Vocm and Vod to zero voltage and no more performance impact to it?!  how about your comments or suggestion about the dull supplies and performance impact?

Thanks so much again and hope your next reply.