LT6658 with voltage divider on output

Hi Engrjhnsn,

The AD7124 MIGHT be able to tolerate the 3.3k || 10k impedance of this circuit - note that it has reference buffers that can be enabled / disabled. Since this is mainly an AD7124 question, I'm moving it to the  Precision ADCs forum.

But a couple of things to note - unless you pay particular attention to the ratio drift w/ respect to temperature of your external resistors, it is likely that they will completely dominate the overall drift of the OUT3 node.  See the LT5400 datasheet and application notes for some insight into this subject.

Also - why not use the AD7124's internal 2.5V reference?

Also - depending on how your thermistors are configured, the measurement is often ratiometric such that reference accuracy and drift cancel out of the equation. Circuit Note 545 goes into great detail on measuring thermistors with the AD7124 specifically.

-Mark

Mark,

The resistors I have are .1% and 25ppm so I was hoping they would be good enough especially as you pointed out with the ratiometric measurement. The CN0545 is exactly what I am following but with the AD7124-8PMDZ board. This will be interfaced to a Raspberry Pi.

I wasn't using the internal reference because with worse case using 1K resistor for my thermistor divider would be 2.5mA per channel x8 so 20mA. The internal can only handle 10mA.

I will see if I can figure out the impedance you pointed out but if I can't then I will look into another reference chip or wait to get a 2.5v version. I did find the LT5400 when looking at other datasheets so I will revisit that one.

You could add an external buffer for the AD7124's internal reference. You could find an op-amp that would do the job, but the LT3040 is specifically designed for this sort of thing. It will handle, and in fact requires, a large output cap (4.7 uF minimum).

Worst-case dropout voltage is spec'd at 310 mV at 50mA, so you should be fine with a 3.3V input.

Always a good idea to test on the bench though - there's an eval board for the LT3040, or you could solder the MSE package to a breakout board. Beware power dissipation if you don't solder the exposed pad down (fine for prototyping, but you'll want to follow the datasheet layout recommendations when you go to production.)

-Mark

I didn't realize that was an option to buffer the refout and I like the LT3040 you pointed out. So my understanding is that I would use the AD7124 refout as Vin to the LT3040 then LT3040 Vout would supply all the thermistors? It's interesting that in the datasheet examples they use the high accuracy LTC6655 to feed it for adding current capacity. One thing I don't see in the spec for it is the tempco for drift. Is that not an issue and is dependent on its source?

I think I found some LT6658 in 2.5v and also the LT1461 in 2.5v. 

Of the 3 options which way would be best? Two are external and one using internal that you recommended. The LT1461 has 50mA limit and little external components and looks good too.

Thanks 

I didn't realize that was an option to buffer the refout and I like the LT3040 you pointed out. So my understanding is that I would use the AD7124 refout as Vin to the LT3040 then LT3040 Vout would supply all the thermistors? It's interesting that in the datasheet examples they use the high accuracy LTC6655 to feed it for adding current capacity. One thing I don't see in the spec for it is the tempco for drift. Is that not an issue and is dependent on its source?

I think I found some LT6658 in 2.5v and also the LT1461 in 2.5v. 

Of the 3 options which way would be best? Two are external and one using internal that you recommended. The LT1461 has 50mA limit and little external components and looks good too.

Thanks 

Yup, LT1461 is a nice part - lots of output current. But is your circuit ratiometric? If so - the voltage reference cancels out of the equation, and your reference RESISTOR is the important part.

JellenieR any thoughts?

-Mark

Yes it will be ratiometric. The resistors I have are .01% with 0.2ppm/C.

So for the voltage reference will any noise cancel out too? I have been doing lots of reading and that seems to be confusing. 

My goal obviously is a higher current voltage reference with the best features. 

edit: I should add that I ended up originally with the LT6658 due to the dual output and being able to do the reference voltage and as well as the supply voltage.

Since the external ref is recommended to be 2.5v and needs to be no more than AVdd -0.1 (3.2v) so I need two separate supplies. Unless there is another way to do it. I was also using the Eval-AD7124-8sdz as a guide.

I could use the ADP150-3.3 for the supply then something like LT1461-2.5 for the reference.

Thoughts?

I'd defer to the experts in this forum if I'm missing something, but 2.5V isn't more than 3.2V, so the AD7124 should be perfectly happy operating from a single 3.3V supply. I have the setup from this writeup: https://wiki.analog.com/university/labs/software/precision_adc_toolbox operating on my bench right now, and just verified that the supply is 3.3V, and the AD7124's internal reference is outputting an accurate 2.5V.

Regarding noise - indeed, in a ratiometric measurement the noise will cancel to a large degree, with some caveats. High frequency noise will be rejected by the digital filter's response. DC and very low frequency noise will cancel nearly perfectly - but noise at a frequency equal to the multiplexed sample rate will not. The reason is the channels are digitized in sequence, so if the reference voltage changes a bit from the sensor reading to the reference resistor reading, it will show up in the calculated resistance.

You can mitigate this effect by sampling faster, and averaging sensor and reference readings in software.

-Mark

Interesting you bring up that lab because I am planning on using that write up for some development work once my new PI comes in. I like the ADI-KUIPER-LINUX idea with all the software built in. 

To be clear what I was referring to with the different voltages is that if I am doing an external reference it is recommended to use 2.5v. It can go to 3.2v if using 3.3v for AVdd. It would be much easier if one supply can be used for both reference and supply.

You are correct that with the 3.3v supply the internal refout will be 2.5v but due to current demand I can't use the internal reference, hence my problem and needing two supplies. Although with the LT3040 you mentioned I could use the internal reference and buffer through it to supply the thermistors.

Just realized you are the author of that lab. That's awesome. Good work there. I'm currently using a PI with the CN0391 and using Python to import a compiled C library to communicate to it rather than use Pyadi.

Re: Kuiper Linux, keep an eye out, there should be a new image releasing shortly at https://wiki.analog.com/resources/tools-software/linux-software/embedded_arm_images . 

If you're talking directly to the SPI port from your C library this likely won't affect you, but there may have been some updates to the AD7124 driver and / or pyadi-iio interface. Pyadi-iio is easy to update, but the driver would need to be recompiled.

Re: using the same 3.3V supply for both ADC and sensor excitation - it should  be pretty straightforward to do a reality check on whether supply noise is affecting your readings. Try powering the ADC/sensors from a quiet, linear (non-switching) supply, observe the noise in both the individual channel readings (voltage across the reference resistor, and voltage across the sense resistor) and see how they compare to the datasheet typicals. And note that for noise measurements, you should replace the thermistor with a fixed resistor with some nominal value, so that ambient temperature fluctuations don't show up in your noise measurement.

Next, run the readings through the ADC reading to temperature calculation, and see what the noise is in "degrees C RMS" (Or degrees F, if you prefer :) )

Then - run the same measurements with your actual 3.3V supply (like the 3.3V rail from the Raspberry Pi?)

Good luck!

-Mark

Thank you for the heads up on the updates coming to Kuiper. I'll keep an eye out for it.

I figured out where I was going wrong with the external reference voltage not being able to do 3.3v. Following the cn0545 lab and ad7124_temperature_measurement_demo they enable all the buffers for REFIN and AIN. This is fine for both internal and external at 2.5v but not above 3.2v if using 3.3v for supply. I kept getting this error when using the online EVAL tool. Once I disabled buffers everything is good.

Do you see any issue with not buffering these inputs?

Once up and running I'll follow your tutorial for noise and do some test.

Thanks

Re: buffered reference - JellenieR , any thoughts?

But looking at the datasheet - reference current in unbuffered mode is only 12 microamps, so the only reason you'd ever need to enable it is if you're deriving the reference from a high-impedance source.

Side note - occasionally you'll see circuits that derive a reference voltage from a current sense resistor. While this works in theory and saves you a bit of math, in practice it's almost always better to driver the ADC's reference pins with a quiet, low-noise source.

-Mark

Hi All,

Apologies, I missed this thread and thanks for tagging me Mark. 

In terms of enabling the reference buffers, these buffers will require a 100mV headroom above AVSS and below AVDD supplies. So if you have AVDD=3.3V and AVSS=GND=0 then each REFIN pins can only accept 0.1V to 3.2V thus the tool flag this as an error since a 0V at REFIN- is outside range. But In general the AD7124 can accept reference voltage up to AVDD when buffers are disabled. 

I'm trying to read the conversations above, and going back to the original question. I think having additional divider would affect your thermistor resistive divider, if in case you are trying the same config as CN0545. So if you want accurate measurements then these resistive dividers must also use high precision/accuracy and low tempco. And since you are driving the reference inputs with high resistor values then the reference buffers must be enabled as these may cause additional gain error. 

Mark suggesting to use the internal reference and you are worried about the limitations of the output current. If you have the same config as CN0545 then you can just probably use a higher value of the sense resistor to limit the current.

Thanks,

Jellenie

Jellenie,

Thanks for joining the conversation.

As you pointed out that is where I wasn't fully understanding Mark with being able to use 3.3v for reference. My settings were wrong using the labs mentioned and enabling the buffers.

I realize the divider on the output of the voltage source is a bad choice. I can't change the divider resistance based on the criteria I need so it is about 1K worst case if the thermistor is maxed out and close to 0 ohms. With that said worst case would be all 8 thermistors at 1K so if using 2.5v that would be 20mA and if using 3.3v 26.4mA and the reason I was looking at external reference.

Yes I have high precision low tempco resistors for this and I think the input would be low impedance since there is only the 1K from divider plus the 1K inline on the board already. Unless I am missing something. 

I guess with all this in mind would it be better to use 2.5v with or without buffers or 3.3v without buffers for the external reference?

Thanks

Hi, 

In general, since thermistor is a resistive sensor, we always tend to keep current through the sensor not too large to ensure that the power dissipation or self heating effect of the sensor will not affect the measurement results. So I think you need to balance the design whether you need to use an excitation voltage or an excitation current. For a 1k thermistor resistor it might be good also to consider another option of using an excitation current rather than an excitation voltage so that only small amount of current will flow through the sensor. The AD7124 have a built-in excitation current that you could use on your application. For a higher resistance values of thermistor (i.e. 10k or above) a voltage excitation may suit here and you can just use the internal reference as excitation voltage. I have here some article that you may find helpful when designing a resistive temperature sensing measurement. 

How to Select and Design the Best RTD Temperature Sensing System | Analog Devices

Thermistor-Based Temperature Sensing System—Part 1: Design Challenges and Circuit Configuration | Analog Devices

Thanks,

Jellenie