Single Ended to Differential Amp (LMH6550)

Hello Santosh,

I would not expect the output to be seen coming from the negative terminal, if you have disabled the top LMH6550. Have you confirmed whether this can only be coming from the top LMH6550 disabled amplifier by removing the 0ohm connected in the negative terminal path and not from the B mux? Also, have you made sure that the signal on the B path is completely turned off? Also, could you probe on an oscilloscope the floating positive node of the top disabled LMH6550 to make sure that there is no signal riding?

Functionality wise, if you are not shorting the amplifier output to ground, the load presented by the adc should be fine.

The output impedance of LMH6550 in disabled state should be on the order of a couple of Mohms.

I would not recommend running the LMH6550 on 5V supply and applying a signal amplitude of 9V at the outputs, as it is beyond the absolute maximum rating of the part. The abs max rating is within supply voltage.

Best Regards,

Rohit

Hi Santosh,

The LMH6550 can tolerate up to 12 V supplies, so if you give a 10V supply the 9V from the Mux will be OK. 

The main issue will then be the signal through the feedback resistors to the amplifier inputs.  The inputs are designed to operate with 0V differential voltage, because of this there are back to back protection diodes bewteen the two input pins.  If the differential voltage is large enough there will be signal feed through on these diodes, this could cause issues with isolation.

Also, it's not a good idea to float one of the outputs, the single ended to differential conversion only works when one of the two inputs is set to a fixed voltage (preferably the same DC value as the other input).

I would also caution against having the two amplifier outputs connected by 0 Ohm resistors if both will be enabled at the same time.  Load sharing resistors of at least 10 Ohms might be useful. 

Regards,

Loren

hi Rohit,
thanks for the response, greatly appreciate it! The circuit is still in concept stage. i haven't designed the board yet. I plan to have only one path activated at once and hence in single ended mode, just one mux should be working. I just wanted to know if the sine wave at the output of the opamp in disabled state would go back to the input and cause some kind of ringing. So in disable mode if the DAC is generating a sine wave, the opamp should not be acting as a load as the input is floating and also, we shouldnt be seeing any sine wave on the negative output, right?

thanks,
Santosh

hi Loren,
greatly appreciate the response! The issue you mentioned about isolation, will that be present even when the opamp is disabled ?
I don't plan to float the outputs. The positive input though will be floating when i select single ended operation. When the single ended operation is selected, i plan to disable the opamp by pulling the enable pin low.

I don't want to use two amplifiers at the same time. I would like to enable only one at a time as they could be fighting when you have two enabled at once.

thanks,
Santosh

Hello Santosh,

First, I think 10k resistors are too big for the LMH6550 it is high speed and does not tolerate large feedback resistors.  (to check you can simulate with TINA-TI)

The isolation issue is present even when the amplifier is disabled.  Because the feedback resistors and isolation diodes are passive they will transfer signal around the amplifier in all cases. 

IIt is for this reason that FDA amplifiers are seldom used in multiplexing applications. 

I am attaching a TINA-TI schematic that you can use to simulate different configurations.  Just position the switches appropriately and then you can see the signal flows.  Use a transient simulation, AC simulations are small signal and will not show how the ESD diodes work under large signal conditions. 

You may want to use TINA-TI to simulate your circuit before you build it.  If you are using the LMH6550 on a single supply you need to carefully consider the DC voltages of the inputs and the signal to make sure they are in the valid ranges for the amplifier (roughly 1V from the rails for inputs and outputs, but check the datasheet because the input is shifted down). 

LMH6550_mux_e2e_full.TSC

Regards,

Loren

Loren,

The isolation issue would be a problem for me in this case.  Can you suggest a part that i can use that would let me to isolate the signal or a different approach ? I want to use the 10k resistors as i dont want to draw high currents from the supply. What kind of issues could we be looking at if we're using 10k resistors?

 

thanks,

Santosh

Loren,
Another doubt that ihave is that though the signal will be seen on the feedback resistors( only positive output) and possibly isolation diodes at the input of the opamp, the opamp shouldnt be amplifying the signal, is that right? and the signal shouldnt be showing up on the negative output in the disable state, right?

thanks,
santosh

Hello Santosh,

The recommendation for feedback resistors is a general observation for high speed amplifiers.  When the feedback resistor is too big the amplifier can be unstable.  I ran some simulations and the LMH6550 may be OK with the 10k resistors, but it is hard to be sure without building a prototype. 

The amplifier will not amplify the signal on the inputs when it is disabled.  The signal path is entirely in the feedback resistors and input protection diodes.  Aside from simulating it in TINA-TI there is no way to be sure without building a prototype. 

One thing to consider is using a buffer (or relay) between the first amplifier negative output and the second amplifier positive output.  This would eliminate the isolation issue. 

Regards,

Loren

hi Loren,
thanks for the inputs. I did simulate in orcard with 10k and it seemed fine. But like you said, there is no way to be sure without building a prototype. I am considering ordering the eval kit associated with this part (LMH730154/NOPB) to check fort he behaviour of the part. One option i am considering is putting a jumper on the negative input of the opamp to prevent any significant leakage to ground. the circuit will be open then and the opamp in disable state. Any thoughts on this ?

thanks,
Santosh

hi Folks,

need your expertise again! I tried to simulate the circuit i was going to build on orcad. the simulation is with the opamp disabled. I have a switch on the positive pin to isolate  the ac signal from going to the input. And i have a AC signal on the positive output pin. Even in disabled state i can see the output on the negative pin. should this be happening?  I've attached the simulation results to show the plots where the probe is placed.

This is basically what i've tried to implement on spice

.

I've also attached the simulation results. Your help is most appreciated!

thanks,

Santosh

 

 

opamp simulations.docx

Hi Santosh,

 

I think that your simulation is correct.  You will need another relay if you want good isolation.  The amplifiers were not designed for this scenario. 

 

Regards,

Loren

 

 

hi Loren,

Where do you suggest adding another relay? on the output pins? Which amplifier do you suggest i use for this scenario ?

 

thanks,

Santosh