A Compact, Ultra-Stable 1V Reference for Precision Constant Current Sources - Page 1
Forum section: electronics / general

author: zlymex
author_rank: Frequent Contributor
timestamp: September 23, 2025, 02:58:28 pm
content: Keywords: Ultra-low noise 1.25V reference; Miniature on-board oven; Low attenuation factor statistical resistor divider; Universal constant current drive; Sub-ppm short time stability current sourcePart 1: PrincipleHigh stable constant current sources are typically generated based on Ohm's Law (I=V/R) using a reference voltage and a precision shunt resistor. To improve short-term stability, a higher reference voltage, such as 1V, is preferred because it results in relatively lower thermal EMF and noise.However, precision buried Zener references often provide higher native voltages, necessitating resistive division. From the perspective of the attenuation factor of the divider on the target voltage, a larger division ratio results in worse attenuation of reference noise/stability, while a smaller ratio is better. Therefore, selecting a reference voltage close to 1V is advantageous for precision division. Fortunately, precise 1.25V references exist, which are near the required 1V and offer the benefit of low-voltage operation. Commercial examples like the LTC6655-1.25 and REF7012 feature ultra-low 0.25 ppm p-p low-frequency noise and are native bandgap references, meaning they do not have internal resistive dividers or boosters. Testing indicated that the former (LTC6655) had higher noise and significant hysteresis in plastic packages. Consequently, the LCCC-packaged REF7012 was selected as the final reference.To further stabilize the voltage, a miniature on-board temperature control (oven) is employed. Although the circuit is simple, its small size results in low power consumption and short stabilization time.A "statistical divider" utilizes a Vishay osopa1001 network resistor, which contains ten identical 1kΩ resistors within a single package, providing excellent stability for voltage division. When configured for a 1.25V-to-1V division, the worst-case individual resistor attenuation factor is 0.1, verifiable using partial differentiation. This means if any single resistor changes by 1 ppm, the 1V output changes by at most 0.1 ppm, thereby enhancing the stability of the division ratio.The following is an example of a 10V voltage division, with a total resistance of 10k and a current of 1mA, divided into 9 cases to output 1V to 9V. lt can be seen that the attenuation factor is smallest when outputting 9V, and is not significant for low voltages. For the1.25V to 1V voltage division, it is equivalent to a 10V to 8V voltage division with a attenuation factor of 0.2, but the network resistors are used in parallel pairs thus the single resistor attenuation factor is 0.1.The circuit schematic includes a final test current source stage. For currents from 1mA to 100mA, an on-board Rs and Q1 (BSS108) are used. For higher currents (up to 10A), Q1 is replaced with an IRF3708 (low Vgs-th), and Rs and RL are connected externally.Part 2: AssemblyThe REF7012 is mounted using flying leads. This method facilitates placement within the miniature oven and helps isolate the reference from PCB stress. The leads are 0.24mm diameter wire-wrapping wire (approximately 23 AWG).The internal dimensions of the oven are only 7mm x 9mm, containing one SOT-23, one 0805, and two 0603 components. After soldering, thermal grease is applied for temperature equalization, covered with a thermally conductive silicone pad. The REF7012 is then soldered on top of the pad. Finally, black foam is placed both the top and bottom for wind protection and thermal insulation.A photograph shows the assembled board, with the current source set for 1mA using 1k metal foil resistors for both Rs and RL.Part 3: TestingAfter assembly, CVCC power is supplied with current-limited to 50mA to prevent damage in case of fundamental errors or soldering mistakes. Using a lithium battery for initial power-on is not recommended, as a short circuit could damage the battery and potentially destroy components.Unless otherwise specified, all voltage measurements were performed using a Keithley DMM7510 multimeter on the 10V or 1V range, with NPLC=5, Auto Zero=On, Filter=Off, Input Z=Auto, and Line Sync=On. All short term noise is represented as ppm peak to peak, which is easy to observe from test charts, and will also not change when resistive divided or boosted, if neglecting the noise of participating resistors and op-amps.Initial checks involve measuring the 2.8V, 1.25V, and 1.00V test points sequentially to verify normal operation. The resistance of the NTC thermistor (relative to ground) is then measured using the 100kΩ range to monitor temperature change and stabilization.Results indicate the temperature control is functional. The temperature rises to near the setpoint within about half a minute, but subsequent stabilization is slower, likely due to less-than-ideal thermal isolation or NTC stress.The following chart shows the 1.25V curve, tested in the 10V range. The overall curve is very stable, with a noise of approximately 3uVp-p, or about 2.5ppm p-p, which is slightly higher than expected, mainly due to measuring a low voltage on the 10V range.The following chart shows the 1.00V curve, tested also in the 10V range.After the voltage tests were completed and confirmed to be normal, the constant current source was tested. First, Rs=RL=1k were soldered to test 1mA, the power source is switched to a battery to eliminate potential ground loop issues from the AC supply. If using an AC supply, the meter's negative lead should be connected to the 1V ground or the supply's positive terminal.Subsequently, two available 80Ω metal foil resistors was used for Rs and RL, yielding a current of 12.5mA. Plots show the voltage across RL for both 1mA and 12.5mA conditions.The short term noise appears even lower than the 1V reference measurement, approaching 1 ppm p-p, due to the use of the 1V range.

author: zlymex
author_rank: Frequent Contributor
timestamp: September 23, 2025, 03:17:02 pm
content: After on-board current testing, a 1A current source was tested. The on-board Q1, Rs, and RL were removed and replaced with an external TO-220 transistor (e.g., IRF3708) and two UNR 4-4020 resistor. The three power devices were mounted on a heat sink.Testing showed rapid stabilization and low noise for the 1A output, similar to the 1mA case, but with some fluctuation (~2 ppm p-p). This is likely due to temperature drift in Rs and RL, given the not-so-low power dissipation of around 1W.A 10A test was then conducted using similar hardware, but with Rs and RL replaced with 0.1Ω resistors of the same series. The power dissipation in these resistors reached 10W (abs. max is 50W), with total system power around 37W, necessitating a fan on the heat sink.Short-term noise observed on the DMM7510 was acceptable (<2 ppm p-p). However, the overall output showed significant fluctuation, characterized by a reverse "jump" followed by slow recovery, later traced to the poor RL resistor.Replacing RL with an AE FNP-Z R1001B resistor eliminated the jump, though the output curve still exhibited a downward drift, likely due to the Rs resistor.For the final 10A test, both Rs and RL were replaced with AE FNP-Z R1001B resistors (with RL housed in a terminal block and Rs simply attached to the heat sink).The initial part of the curve remained unstable, requiring about 20 minutes to settle. However, the short-term noise was low, confirming the voltage reference itself was not the issue.Part 4: Follow-up and ReferencesMeasurement cycle duration and noise.As mentioned earlier, testing used NPLC=5 with line synchronization, resulting in a measurement cycle time of 0.36 seconds. The actual measurement time is 0.1 seconds for NPLC=5, with the remaining 0.26 seconds used for auto-zero and synchronization.For low-noise measurements, the Keysight 3458A is often used with NPLC=100, resulting in a 4-second cycle (2 seconds measurement, 2 seconds auto-zero). A statistical rule states that under identical conditions, noise is inversely proportional to the square root of the measurement time. Thus, if noise is 1 for a 0.1s cycle, it becomes approximately 0.316 for a 1s cycle and 0.1 for a 10s cycle.Therefore, the DMM7510 PLC=5 cycle time is 1/20th that of the 3458A PLC=100, implying its noise would be √20 ≈ 4.4 times higher. Consequently, if measured with the 3458A at PLC=100, the observed 1mA noise of ~1 ppm p-p would reduce to about 0.3 ppm p-p, firmly in the sub-ppm range. This explains why Reference 1 below uses the 3458A at NPLC=100.

author: zlymex
author_rank: Frequent Contributor
timestamp: September 23, 2025, 03:24:23 pm
content: Reference 1: A 10-A High-Precision DC Current Source With Stability Better Than 0.1 ppm/hThis paper uses a Fluke 732B 1.018V reference directly. Rs and RL are custom 50W, 0.1Ω resistors from Tinsley. Measurements used a 3458A at NPLC=100, reporting 0.5 ppm peak-to-peak noise, with 0.1 ppm being the RMS stability over one hour.Link:https://docs.ampnuts.ru/eevblog.docs/_Metrology/wang2014.pdfReference 2: High Stability Current Control in the 10A RangeThis design uses a 5Vbandgapreference (LT1027 with 0.6 ppm p-p noise), deriving a 2V reference through division.Link:https://rubiola.org/pdf-articles/conference/1994-cpem-precision-current-control.pdfReference 3: Design and Evaluation of a 10-mA DC Current Reference StandardThis reference uses an LTZ1000, similar to the Datron 7001, employing a "statistical" method to achieve 10V, with Rs=1kΩ.Link:https://cds.cern.ch/record/643294/files/cer-002399331.pdf

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2025, 01:39:46 am
content: I wonder how they got such low noise out of a bandgap reference. It looks like they used a much higher quiescent current (5 to 7 milliamps) to do it; comparable buried zener references achieve almost the same noise at less than 1/4th of the current, but their cost means you sure would not want to use 4 in parallel for even lower noise.The LT6654 series is the same thing, but much lower power, so half the accuracy and 3 times the noise and drift, but more suitable for battery powered instrumentation.

author: zlymex
author_rank: Frequent Contributor
timestamp: September 24, 2025, 09:42:04 am
content: Quote from: David Hess on September 24, 2025, 01:39:46 amMy guess is they use multiple parallel, but no die photo found. The closest availablebandgapmaybe theMAX6350, which contains multiple repeating patterns.Edit: Other low-noise bandgaps I found include the ADR4525(confirmed in the datasheet, 0.50 ppm p-p) and MAX6126/6226(0.57 ppm p-p). Their quiescent currents are all much lower than those of the REF7012 and MAX6655.

author: macaba
author_rank: Regular Contributor
timestamp: September 24, 2025, 09:48:35 am
content: Thanks, I always enjoy seeing your work, and it looks like a good result.

author: Andreas
author_rank: Super Contributor
timestamp: September 24, 2025, 11:04:20 am
content: Quote from: zlymex on September 24, 2025, 09:42:04 amSorry but MAX6350 and LT1027 are buried zener references.with best regardsAndreas

author: zlymex
author_rank: Frequent Contributor
timestamp: September 24, 2025, 11:39:51 am
content: Quote from: Andreas on September 24, 2025, 11:04:20 amThanks for the correction.

author: exe
author_rank: Unknown
timestamp: September 24, 2025, 04:49:10 pm
content: > From the perspective of the attenuation factor of the divider on the target voltage, a larger division ratio results in worse attenuation of reference noise/stability, while a smaller ratio is betterCould you please elaborate that? I thought larger division would attenuate both voltage and noise. For the same reference noise, downscaling 6.9V, I think, should give better result than downscaling 1.24V.

author: ch_scr
author_rank: Frequent Contributor
timestamp: September 24, 2025, 04:58:34 pm
content: The way I understand it, it's a question on the "balance of parameters". Sure, voltage noise, tempco and ref drift will be attenuated.But if you look at the e.g. 0.2ppm/°C tempco of the heated ref, now 70% of that will depend on the resistor divider!One will be hard pressed to get a divider that won't degrade that. Similar story for the long-term stability.

author: zlymex
author_rank: Frequent Contributor
timestamp: September 24, 2025, 05:37:09 pm
content: Quote from: exe on September 24, 2025, 04:49:10 pmNoise and tempco won't attenuate through dividers if represented in relative terms such as ppm p-p or ppm/K.Attenuation factor is Weakening Factor describes how a variation in a single resistor in the network (osopa1001) is reduced in its effect on the divided voltage (1V). In my case, when a single 1k varies 10ppm, the 1V only varies 1ppm at most.

author: EC8010
author_rank: Frequent Contributor
timestamp: September 25, 2025, 02:39:21 am
content: Quote from: zlymex on September 24, 2025, 09:42:04 amHow do you know that MAX6126 is a bandgap reference? I didn't see any comment about the technology except a MAX6226 data sheet BiCMOS note. Could they perhaps be X-FET?

author: RolandK
author_rank: Regular Contributor
timestamp: September 26, 2025, 07:58:41 am
content: V1 and RL are in series.If you exchange them, RL and RS are in series, too (except for the sense-circuit). This way you can make relative mesurements of RL to RS if you want to transfer resistor calibrations.

author: ArgyllGargoyle
author_rank: Contributor
timestamp: September 27, 2025, 01:37:03 am
content: Has Noopy decapped max6126? You might find the answer to this question if you saw the max6126 die. Would also be interested in seeing ltc6655

author: gamalot
author_rank: Super Contributor
timestamp: September 27, 2025, 08:19:17 am
content: QuoteIt seems like you might’ve meant LTC6655 instead of MAX6655.

author: zlymex
author_rank: Frequent Contributor
timestamp: October 08, 2025, 11:34:02 am
content: Quote from: EC8010 on September 25, 2025, 02:39:21 amIt's a guess based on the fact that many band gaps (such as LT1009, AD584, ref5010) as well as MAX6126 have parabolic shape in room temperature for their tempco curve, while ADR420 and ADR430 of ADI have not. Although MAXim is now part of ADI but MAX6126 produced before the deal.

author: zlymex
author_rank: Frequent Contributor
timestamp: October 08, 2025, 11:36:10 am
content: Update Circuit Diagram

author: zlymex
author_rank: Frequent Contributor
timestamp: October 08, 2025, 11:39:06 am
content: How to accurately measure the noise and short-term stability of a constant current source? The conventional method involves using a load resistor (RL) with a resistance similar to Rs and measuring the voltage with a high-resolution DMM. While increasing RL can improve measurement resolution, the resulting power dissipation leads to diminishing returns.The back-to-back current measurement method proposed in Reference  can nearly eliminate the influence of RL - by selecting an RL value 1000 times higher than usual, this approach not only significantly enhances sensitivity response but also ensures that the minimal voltage drop across RL results in negligible thermal effects from power dissipation.In my setup below, two identical 12.5mA sources were configured in a back-to-back arrangement with RL set to 100 times Rs (i.e. 8kΩ). Using the DMM's 1V range, this configuration achieves 100x sensitivity enhancement; switching to the 100mV range further improves sensitivity by 1000x. With total power dissipation below 1uW, thermal effects and i/f noise become negligible. Measurements were performed using a DMM7510 (with integration time PLC=5) yielding a 15-minute peak-to-peak voltage of 60uV (equivalent to 7.5nA peak-to-peak current). As this value represents the combined output of both sources, the single-source equivalent is obtained by dividing by √2 - resulting in 5.3nVp-p or 0.43ppm p-p current noise, three times improvement from the previous measurement.

author: maat
author_rank: Regular Contributor
timestamp: October 09, 2025, 08:31:32 pm
content: Quote from: zlymex on October 08, 2025, 11:39:06 amDo note, that this measurement technique relies on R_{out} >> R_L || R_DMM. It therefore produces incorrect results at frequencies above DC, when the output impedance of the current sources drops. The 10 Hz sampling is fine in your case though.

author: ivo
author_rank: Regular Contributor
timestamp: October 13, 2025, 09:38:46 am
content: Quote from: EC8010 on September 25, 2025, 02:39:21 amBecause for instance the MAX6126 2.5V, operates off a 2.7V supply, and/or has <0.1V dropout voltage. No one is building a building a 2.5V buried zener, I'm not really aware of any built that aren't a ~5.1V (or higher) zener + transistor for tempco, making them practically all 6-7.5V floor to run the inside reference. Note MAX6126 was made way before AD acquisition of Maxim. Practically every single zener reference requires you to give it 7.5V at least, whatever its output is