LT6658 with voltage divider on output

Question

Is it ok to add a voltage divider on one of the outputs of a LT6658 voltage reference? This would be a 3.3v chip and I need 2.5v for an external reference to an AD7124 and feed the multiple thermistors. Doing it this way I can get away with only 5v source to LT6658. I don't know if the voltage out would still be considered stable reference. The other output would be the 3.3v supply to the AD7124. 
 The other option is to try and find a 2.5v version of the chip then boost one of the outputs like the datasheet shows but then I would need minimum 5.8v to get the 3.3v needed for AD7124 supply. I do have 12v available so that is an option if I can get one. 
 I tried to run in LTspice to see if there is any issues but I am not familiar enough with the software. I just see 2.48v out.

JellenieR · Accepted Answer

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 t hese 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)

JellenieR · Answer

Hi 
 Thanks for your comments. With regards to the Rsense, the reason I designed it that way is to ensure that the output voltage of the Rsense and Rth at the base temp of 25C is always at the midpoint at nominal temp. But if you need to change it to meet your requirements then I see no harm. I just did not allow it in the tool as this would complicate things in terms of calculations for limitations. But we'll see for future revision, I'll take note of this as consideration. 
 With regards to your thermistor resistance, I thought you are using 1k thermistor that is why I recommended an excitation current. If it is 10k then a voltage divider is more appropriate. So I guess going back to your original question, it could be safer if you use 2.5V as if you use 3.3V you need to ensure that this 3.3V is very accurate and could not be higher than your supply voltage. I know voltage reference has also limited supply current so it is very unlikely to use it as both supply and voltage reference. 
 Thanks, 
 Jellenie

MarkThoren · Answer

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

MarkThoren · Answer

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

MarkThoren · Answer

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

MarkThoren · Answer

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

MarkThoren · Answer

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

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LT6658 with voltage divider on output

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