Hi

I am currently designing a circuit where I need to split a single 0V-5V analog signal (coming from a DAC chip) to go to 8 separate IC's. The 8 IC's are on separate PCB's that are connected with a long cable (about 4m max) to the PCB with the DAC. I need the signal that reaches all the IC's to be identical.

I was thinking of using unity gain buffers in the form of a unity gain op-amps. I selected an op-amp that has rail to rail capabilities (so that I don't need a negative supply) and is unity gain stable. Also note that this op-amp chip has 4 elements.

So here is my question: Do I only need to use 1 op-amp and then split the signal wires from the output, or do I need to use 8 op-amps with the input from the DAC splitting to each of the op-amp inputs and each of the outputs going to one of the 8 IC's? You can look at the attached schematics for the 2 options. I would greatly appreciate any help.

 

Will this signal change only slowly or will it have fast transistions?
What is the input impedance of the eight inputs?

 

If those PCB's are widely separated and connected with long cables than you should consider converting the signal into a two wire differential signal and receive the signal on each board with a differential instrumentation amp.
That will avoid problems of ground loops or ground noise between the boards from affecting the signal.

 

If you are using shielded cable, like coax, select your OpAmp based on its
ability to drive all the cable capacitance.

The MCP6064 is only characterized for 60 pF load, that may not be enough to maintain
good phase margin, eg. maintain stability.


Regards, Dana.

 

Will this signal change only slowly or will it have fast transistions?
What is the input impedance of the eight inputs?

It is a very slow changing signal. In fact it will be constant most of the time. It will be used to control a constant current supply.

The datasheet of the constant current supply does not state the input impedance unfortunately.

Thanks for the suggestions and I will keep them in mind. I still want to know if the solution makes sense and if I need to use one op-amp for every signal or only one op-amp and then split the signal. If i use one op-amp to drive each of the signals, it should help to reduce the capacitance load on each op-amp like as mentioned by Dana right?

 

The datasheet of the constant current supply does not state the input impedance unfortunately.

Can you give us this datasheet?

 

Can you give us this datasheet?

Here it is. maybe I missed something, i'm quite new to electronic design.

 

Here it is. maybe I missed something, i'm quite new to electronic design.

The analogue pin needs up to 0.2mA.

 

The datasheet implies an input impedance of 25K. That is high enough to allow all 8 inputs to be driven from a single opamp output - but ...

I vote for having a dedicated buffer per output. Besides the issue of driving the capacitance of a long coax cable, it eliminates a single point of failure. It also prevents a possible power supply failure or wiring problem from taking down the control signal to the other supplies. The questions back to you are:

1. Are there any negative consequences of one or two supplies losing their control signal while the rest continue to function normally?

2. Is it more important that some downstream systems stay up during a partial failure, or that the 8 systems are either all up or all down together?

ak

 

The datasheet implies an input impedance of 25K. That is high enough to allow all 8 inputs to be driven from a single opamp output - but ...

I vote for having a dedicated buffer per output. Besides the issue of driving the capacitance of a long coax cable, it eliminates a single point of failure. It also prevents a possible power supply failure or wiring problem from taking down the control signal to the other supplies. The questions back to you are:

1. Are there any negative consequences of one or two supplies losing their control signal while the rest continue to function normally?

2. Is it more important that some downstream systems stay up during a partial failure, or that the 8 systems are either all up or all down together?

ak

Thanks for your feedback. to answer your questions: It is more important that at least some of the outputs continue to function even if one fails. If one fails, the overall system would still be able to function until repairs can be made. It will thus be advantages to have one op-amp per signal for that reason.

So just to be clear: The only other benefit of having one op-amp per signal is the ability to drive longer cables?