We have a pressure sensor that outputs 0 - 5 volts, with a need to feed a SENT (Single Edge Nibble Transmission) message generator (ZSSC4161D) with a differential voltage centered around 2.5 volts, as though the input voltage was coming from a Wheatstone Bridge. Is there a relatively simple circuit that could perform this conversion?

 

with a differential voltage centered around 2.5 volts

Not clear what that means.
So exactly what outputs do you want for a sensor voltage of 0V, 2.5V and 5V?

 

The SENT device requires a differential input voltage centered around 2.5V, which is what you would expect from a Wheatstone Bridge type of sensor. So, with zero pressure measured by a Wheatstone bridge type of sensor, the two output voltages would both be 2.5V. If the pressure sensor saw half-range pressure, the output voltage would be something like 2V and 3V, or a differential of 1 volt, centered around 2.5V. Full scale pressure might give 1.5V and 3.5V differential voltages, or a differential of 2 volts.
The objective here is to convert the 0 - 5V signal covering the full sensor pressure range to a differential centered around 2.5V.

 

Sounds like a job for an op-amp or two. How rapidly does this pressure sensor output change?

 

Below is the sim of circuit using two op amps (one package, inexpensive) with rail-rail outputs that gives a differential output centered at +2.5V:
The outputs go from +2.5V to +3.75V (yellow trace) and +1.25V (red trace) for an input of 0V to 5V (X-axis).
The differential output (difference between the two outputs - green trace) thus goes from 0V to 2.5V.

The 2.5V output offset value does depend upon the value of the 5V supply (varies by 1/2 the 5V change), but that doesn't affect the differential output voltage (differential gain), which is determined by the resistor ratios.

The output bandwidth should be over 500kHz.

Is that satisfactory for your purposes?

 

That is exactly what we need ! ! ! Thank you. I never would have come up with that circuit...

 

That is exactly what we need ! ! ! Thank you. I never would have come up with that circuit...

Great.
Let us know how it works if you build it.

 

I assume you designed then simulated the circuit to produce the graph. If that's the case, I will breadboard it before committing it as a small part of a much larger PCB! Will let you know how it goes once I get a couple of LMV358A's.

 

Another solution is to use a single ended to differential op-amp.
I would use an LMH6550 to do that.

 

It’s a differential input, so it isn‘t going to be bothered what the common mode voltage is.
Connect the signal to BR1P, and bias BR1N to 2.5V with a couple of resistors or a 2.5V reference.

 

I assume you designed then simulated the circuit to produce the graph. I will breadboard it before committing it as a small part of a much larger PCB!

Yes, it was from an LTspice simulation.
And yes, you should always breadboard a circuit before committing to a PCB.

Will let you know how it goes once I get a couple of LMV358A's.

You do know there are two op amps in each IC package(?).

 

Another solution is to use a single ended to differential op-amp.
I would use an LMH6550 to do that.

That chip should work but it's significantly more expensive (about a factor of 10) than the op amp IC I used (if cost is a factor).

 

It’s a differential input, so it isn‘t going to be bothered what the common mode voltage is.
Connect the signal to BR1P, and bias BR1N to 2.5V with a couple of resistors or a 2.5V reference.

That should work, depending upon the common-mode range and common-mode rejection of the circuit (which I could not find in the data sheet).

 

That should work, depending upon the common-mode range and common-mode rejection of the circuit (which I could not find in the data sheet).

Provided that he's not taking it a huge distance.

 

The frequency range is very low (forgot to answer your question). It's monitoring an automobile MAP sensor, so it moves relatively slowly (100Hz, not MHz). Yes, I realized the LMV358A is a dual amp.
I was told by the Renesas people that the input has to be centered around 2.5V, even though I would tend to agree with you that the input should support a common mode voltage. I find the specs for these products quite lacking for the uninitiated.

"...It’s a differential input, so it isn‘t going to be bothered what the common mode voltage is.
Connect the signal to BR1P, and bias BR1N to 2.5V with a couple of resistors or a 2.5V reference..."
Actually, I am going to try your suggestion "with a couple of resistors" so BR1N is tied to 2.5V since I have the evaluation board for the ZSSC4160 series. I am waiting for them to send me the proper code to drive the board with their GUI. Lots of hurdles with this project!

Thank you for doing the research into the Renesas chip!!!

 

I find the specs for these products quite lacking for the uninitiated.

They are quite lacking in specs for anyone (initiated or not) who wants to use the chip.
There should be a complete spec sheet available, but they only seem to post a short-form one.

 

The specs are limited in distribution "to protect themselves and their customers". That was one of the hurdles I referred to; getting approved before I could see the technical documents. Once I got the "detailed" specs (not so much) I found this comment about the inputs: "Symmetric behavior and identical electrical properties (especially the low-pass characteristic) of the differential bridge sensor inputs are required. Unsymmetrical conditions of the sensor and/or external components connected to the sensor input pins can generate a failure in signal operation."
Also; "Main signal path polarity: The resistive bridge sensor element signal is input via the BR1P and BR1N or BR2P and BR2N
pins and is handled as a fully differential signal. Both signal lines have a dynamic range symmetrical
to the common mode potential (analog ground; equal to VVDDA/2) so that it is possible to
process positive and negative differential input signals. These differential signals are pre-amplified
by the programmable gain amplifier (PGA) and are converted to digital values by the analog-to-digital
converter (ADC)." I find it somewhat difficult to understand what they are saying here! Only if you are curious, refer to the attached, which you may not have seen.
They say you can load the parameters using the GUI or by I2C communication, BUT, they give no examples or guidelines on how to do the latter. It is reasonable to set up a few chips using OWI from the GUI, but for high volume production, I would want to set the parameters from within the application program during setup. No help there.
Only if you are interested, I have attached the specs document, but I think testing is the only way to be ascertain true behaviour.
Page 14 has the table below:

 

Page 14 has the table below:

The Maximum Input Span parameter (mv/V) doesn't make sense to me.
Do you understand that?

 

It looks to me like a weird way of expressing the gain.
PGA * maximum input span = 800
I'm guessing that it means the amount of voltage on the input to give 1V on the output.

Common mode range is understandable - Anywhere between 5% and 95% of Vdda will work, so biassing one input to 2.5V will work; and it will still work with a bit of gain from the PGA.

 

The Maximum Input Span parameter (mv/V) doesn't make sense to me.
Do you understand that?

The way I understood this: I think they mean the span of the differential input as a portion/percentage of the Vvdda, which I assume can be equal to Vcc (5V in our case). With a gain of 1, it allows the range to be 0.5V to 4.5V. With a gain higher than 1, then the span reduces accordingly. BUT, I could be totally wrong, hence my frustration with their documentation.
I had a conference call with Renesas 2 weeks ago, and they said once I get the proper html file that is required to drive the evaluation board, they will walk me through the calibration process if I have trouble. I am still waiting for the file.