ADR1001 + LTC2500-32 measuring module - Page 1
Forum section: electronics / general

author: Korchahin
author_rank: Regular Contributor
timestamp: July 07, 2025, 04:07:26 pm
content: Hi everyone,I suppose many of you have experienced or are currently experiencing the "reinventing the wheel" phase.I recently became fascinated with metrology and related research. Electronics is my hobby, but I'm quite limited in terms of resources and financial investments (for now, my wife still believes I'm buying some cheap old junk).Inspired by the CERN HPM7177 ADC project, I initially considered replicating it. However, it's quite expensive and technically complex for amateur reproduction.On the other hand, I've noticed the ADR1001 reference chip significantly simplifies obtaining a reference voltage for an ADC. It reduces the number of required components, conveniently providing a resistor network, buffer, and voltage reference all within a single package. This naturally encourages simpler applications of this chip.My idea:I need a simple module that includes an LTC2500-32 ADC (+ SPI isolator) and an ADR1001 voltage reference. This module will be mounted onto a separate motherboard.The motherboard might vary depending on different tasks:* Usually, it will include a power supply for the measurement module, an ESP32 microcontroller for data conversion and transmission, and perhaps some additional control (such as thermal chamber control or divider switching).* Alternatively, other motherboards might include DACs for calibration voltages or fulfill other wet ideas.Thus, the module should have inputs for analog power supply (+/- 15 V), SPI signals, and a +5 V digital power line.On the other side, the module should provide a 3-pin differential input (Neg, Pos, GND) for the measured voltage.The goal is to strike a balance between simplicity and maximum achievable resolution.I have limited experience using EasyEDA for PCB design and typically order my PCBs from JLCPCB. I also have some experience designing analog circuits, such as DACs and audio-frequency amplifiers.With this introduction, I'm starting to gather information and recommendations.Questions at hand:1. Bipolar Reference Voltage:The datasheet recommends using an external op-amp to generate the negative (-5 V) reference voltage. However, the positive voltage is buffered internally within the ADR1001 chip. This raises a concern about temperature conditions being uneven for positive and negative voltage buffers. Should I instead consider using a dual op-amp externally to buffer both positive and negative reference voltages in one package to ensure consistent temperature conditions? Or would adding an external buffer for the positive reference introduce significant additional noise?

author: iMo
author_rank: Super Contributor
timestamp: July 07, 2025, 05:23:01 pm
content: Why do you need the -5V Vref?The LTC2500 does not need -5V Vref, afaik. What would you need is an analog front end (AFE), converting let say +/-20V input into the differential input of the LTC2500 (its IN+ and IN- are always within 0..+Vref).. Btw., the most difficult part of your module IMHO..So, as it has been discussed many times here: for the +/-20V (an example only) AFE you will need several suitable opamps, quality resistors, and a floating +/-25V power supply. Most probably AFE will be bootstrapped.The output of the AFE is a differential signal, with LTC's IN+ and IN- input levels between 0V and +Vref (ie. +5V).Also the AFE's output has to be low impedance (as the LTC's IN+ and IP- inputs require low impedance sources).The same for the Vref input, let us assume the ADR1001's 5V output is low impedance..

author: Korchahin
author_rank: Regular Contributor
timestamp: July 07, 2025, 06:40:48 pm
content: Oh, how could I have made such a mistake!It seems I'm still recovering after dealing with the ADS1263 project.Thanks for bringing me back to orbit; I'll now focus more clearly on other critical aspects.Currently, I'm aiming to focus on measurement ranges up to ±12V, suitable for measuring various voltage references and zener diodes. I suppose this range will also be more comfortably handled by rail-to-rail op-amps.Initially, I planned to implement the Analog Front End (AFE) on a separate motherboard. Or do you think it's better to integrate it directly onto the same PCB as the LTC2500? My intention is to fit the measurement module into a compact, temperature-stabilized chamber.Another idea was to measure the ambient temperature of the motherboard and periodically recalibrate the input circuitry when temperature fluctuations occur. Alternatively, introducing temperature-dependent correction factors for the input stage might be useful. For this purpose, the measurement module could also output a buffered version of the ADC's reference voltage for calibration purposes. What do you think about this approach?My ultimate goal is to perform these experiments and developments in modular stages. Having a single measurement module that can be easily transferred between different motherboards, I intend to achieve scalability and simplify development.I'm still studying the datasheets and diving deeper into various nuances, but I'm confident you'll help me navigate this challenging journey of self-improvement!

author: iMo
author_rank: Super Contributor
timestamp: July 07, 2025, 07:04:58 pm
content: The ADS1263 is the same case when talking input levels and AFE (as the most monolithic ADCs).Couple designs here were made with the multislope ADCs, which are different in that regard.It is an interesting project you want to start, indeed, what needs to be elaborated is the AFE, that is still an open question, imho. I have not seen a suitable "24bit capable" solution yet, tbh (despite many ideas and simulations we made during last years).I would put the AFE onto the same board as the ADC and Vref (and the opto SPI isolator as well). Like a 100x100mm 4 or 6 layers pcb. The voltage regulators outside the board (heat source).PS: the AFE should be high impedance input as well, like >XXGohm, with overvoltage protection..Like (for example) a floating OPA140 jfet opamp at the input (with bootstrapped Vcc and Vss you would need a second opamp plus some transistors etc.), with bootstrapped low leakage diode protection at its input, and a stable low ratio TC resistive divider by aprox 10 followed by 2 opamps - output buffers (in order to get low impedance +/-2V diff for the LTC's IN+ and IN- inputs). Or something like that.

author: iMo
author_rank: Super Contributor
timestamp: July 07, 2025, 08:08:29 pm
content: ..As the first step look at this pcb.. Perhaps you may find a schematics too..https://www.eevblog.com/forum/metrology/adi-admx3652-6-digit-20-v-dvm/msg5805127/#msg5805127PS: it seems to me (plus minus) the AFE is using the AD4522 (a chopper) as the input hi-Z buffer (powered by +/-24V, not bootstrapped), followed by couple of AD4528 (choppers powered by +/-2.5V) output buffers feeding the low-Z AD7177's inputs (the same diff inputs as the LTC2500's one). In between is an 8pin resistive divider (perhaps an LT5400, <1ppm/C tracking TC)) with removed markings. The 3pin packages near the AFE's input could be the BAV199 dual low leakage protection diodes.There is a bunch of ADG switches (switching between input CH1 and CH2, etc.) and the AFE's input hi-Z buffer is made twice (dual channel).

author: Korchahin
author_rank: Regular Contributor
timestamp: July 09, 2025, 08:34:28 pm
content: Yes, I understand that the most difficult thing is protection and limitation of the input range from excessively high voltage. Perhaps the protection will be absent in the measuring module itself and will have to be implemented on the motherboard. At the same time, I will get the opportunity to conduct research for different circuits. While studying the AFE topic, I remembered that I once made a circuit for capturing a signal and transmitting it over a differential line. Here is a sketch from memory. I wonder if this can be used somehow?

author: iMo
author_rank: Super Contributor
timestamp: July 09, 2025, 09:14:11 pm
content: It should work such when the AFE's input will be between, say -12V and +12V, the IN+ will be between 0..Vref and IN- will be between 0..Vref as well, and "mirrored" around Vref/2. So neither IN+ nor IN- have go below 0V or above Vref..Also the AFE's IN- and IN+ outputs must be low impedance (required by IN- and IN+ inputs of the ADC).Search for "AFE" here, there are some ideas people discussed already..PS: ideally you may try to reverse engineer the AFE of the above ADI's module..

author: Korchahin
author_rank: Regular Contributor
timestamp: July 09, 2025, 09:26:43 pm
content: Aaaah! I'm looking at this in the datasheet)The reference point should be 1/2*Vref. And what about an input buffer with an active divider? Is this applicable or will it have more noise than a simple resistive divider? In turn, an active divider provides low output impedance...

author: iMo
author_rank: Super Contributor
timestamp: July 09, 2025, 09:33:54 pm
content: Yes, exactly, it is differential around the Vref's midpoint.The divider - the simplest solution is to have a high_Z input buffer (an opamp), followed by the resistive divider (low impedance one like 18k/2k for +/-20V input) and followed by differential output driver (two opamps) with XX ohm output impedance.For +/-20V input the input buffer has to be bootstrapped (needs an additional opamp plus some transistors etc) and say +/-24V power.For +/-10V it may work with simple opamp and +/-15V power.PS: also mind the AFE has to be 6++ digits capable.. People here will tell you what does it mean in practice..