adc zener componsation - Page 1
Forum section: electronics / general

author: hsn93
author_rank: Regular Contributor
timestamp: June 17, 2019, 08:09:57 am
content: hello,i've adc circuit as following (going to 3v3 mcu):https://assets.nexperia.com/documents/data-sheet/BZT52H_SER.pdfmy expectation:voltage divider ratio: 0.5291v -> 0.5v5v -> 2.66v6.24v -> 3.3vin reality:1v -> 0.53v5v -> 2.32v6.24v -> 2.55v15v -> 3.2v18.2v -> 3.3vmy knowledge and understanding:i have zener leakage current:so i took the above graph at 2V i've reverse current of 0.1mAif i have 2v at zener this means (R44 // R29) would draw 0.8mAso Vr43 + 2V = 0.9mA * 2.2k + 2V = 3.98Vin normal calculations = 2/ 0.529= 3.78Vsounds good and working..==============================however,when i do another value which is 3.3Vfrom graph i think it draws reverse current of 5mA , (R44//R29) would draw 1.32mAso total current = 6.32mAVin voltage = 6.32*2.2 + 3.3 = 17.2Vwhich is also fine if i look into the table the zener voltage is actually from 3.23 to 3.37Vhow would you do this componsation? hardware, software?i mean obviously if i have 3.3 in my microcontroller, i should know that this is not (6.24V) but (17V). would i think people tend to componsate the IV curve linear at Q point only right? should we do two point linear componsation?how would you right code, or componsate this?

author: dietert1
author_rank: Super Contributor
timestamp: June 17, 2019, 08:40:49 am
content: I remember a project where i found out about high leakage current of low voltage zeners only after having finished a prototype run of 50 boards. That reverse current depends on temperature.If in your case the zener is a protective device, you may eventually replace it by two shottky diodes to Gnd and to a 3.3V supply.As far as i understand you won't get a reasonable measurement above 6 V with your 2:1 voltage divider, whatever you do.Regards, Dieter

author: hsn93
author_rank: Regular Contributor
timestamp: June 17, 2019, 09:56:19 am
content: Quote from: dietert1 on June 17, 2019, 08:40:49 amhi, thank you.. i remember i get two comments about using zener diodes in previous posthttps://www.eevblog.com/forum/beginners/mcu-adc-circuit-protection/anyway i dont know what you mean with reasonable measurments above 6V.you might be right with (temperature and JN junction stuff) im really not expert..but i set up 8bit software and playing with voltage to my circuit using programable power supply get these results:(SAMD21G) ADC 8bit setup -> 13mV resolution -> with my divider ~24.57mVVoltage Input Value [0-255]measured resolution (V / value) 00NA10042520082530012254001625500202510004124.420008224.4250010224.5300012224.6350014025400015625.6450016926.6500017927.9550018729.4600019430.9650020032.5700020534.1800021437.41000022644.21500024661note that after 6V the resolution increases faster because of characteristics of IV curve. (yes maybe temperature thingy i really know how bad is it)...which makes ADC acting like not linear..so i have this idea in mind.to componsate like what they do in (diode modelinghttps://en.wikipedia.org/wiki/Diode_modelling) with two points (1 for low and another for higher than 6V) for example. what do you think?

author: dietert1
author_rank: Super Contributor
timestamp: June 17, 2019, 10:09:14 am
content: If you are after the nonlinear behaviour, you may first compare several of those zeners to see if their differential resistance is something you can rely on. If that changes from diode to diode or with different makes, your results may be undefined above zener threshold. Or you may need a special calibration for each device.Regards, Dieter

author: Yansi
author_rank: Super Contributor
timestamp: June 17, 2019, 10:21:50 am
content: The solution is to NOT ever use a zener in a circuit like this! I see this mistake over and over.Use a diode clamp instead. Clamp the voltage to safe levels, using a diode pair. For example one clamps to ground (-0.6V respectively) and the other to Vdd (say Vdd+0.6V). These voltage are safe for the succeeding electronics.When a correct current limiting resistors are put in series, this solution is pretty robust. and can tolerate pretty high sustaining overvoltage.Use always a silicon (not schottky) diode (for example 4148, BAV99, ...). Schottky will have higher leakage current again (expecially those higher current ones, and also a good deal of capacitance).Relying on zener diode characteristics and trying to compensate it is pretty bad idea that won't cut the mustard.//EDIT: You may find also this helpful:http://www.ti.com/lit/an/slva898/slva898.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: June 17, 2019, 03:43:25 pm
content: The zener solution is not good - especially zener diodes other than 5-7 V have usually quite some leakage current even below the nominal voltage.A diode clamp to the supply is often sufficient, but not always. Especially the low leakage diodes have too much of a forward drop to be safe for all chips. So with a normal diode is usually needs an additional resistor between the clamp and the ADC to be on the safe side.Even then there may be current through ADC internal parasitic diodes that may effect the ADC / neighboring channels.To avoid this it would need clamping to a voltage a little below the supply voltage.