- Motivation
- Original publication and design files
- xDevs circuit variant implementation, Rev 1.2
- Initial benchmarks and validation
- Conclusion and summary
Motivation
Marta Musso, Massimo Ortolano and Michele Perotti developed and published open-source design [DOI: 10.5281/zenodo.18377393] of transresistance amplifier (TRA) called QuTRA under CERN-OHL-S-2.0 license. This interesting design is tailored for applications and experiments in precise resistance metrology. A transresistance amplifier (also called a transimpedance amplifier, TIA for alternating currents) is an amplifier that converts an input current into an output voltage. Such amplifiers are particularly useful when the signal source like some sensors produce current. Examples of such sensors are photodiodes, ionization detectors, electrochemical sensors. Also TRA and TIA useful to measure and test current-output DACs and in many other precision current measurements.
To mitigate the gain errors and noise sources common in single-stage TRA/TIA setups, the core of the described QuTRA circuit relies on a composite four-stage amplifier structure. The input amplifier maintains a virtual ground at the input node with minimal parasitic bias current to avoid minimize input node voltage error. Then current converted to voltage and amplified with composite gain stages wrapped in a global feedback loop. The QuTRA explicitly accounts for parasitic behaviors that typically degrade TRA circuits. In actual circuit traditional feedback element is substituted with its structural leakage resistance of high quality capacitor/resistor network. To achieve gain stability within system operational bandwidth, QuTRA implements a compensation T-network built out of (R15 and R16) linked to a compensation capacitor array (20 pcs 10 nF C0G C3 to C22). Jumper switch J8 also allows user to directly short out R15. This modifies the equivalent impedance between the input and output nodes for tuning the amplifier’s loop-gain response to fit specific external resistance standards use.
For a precision TRA, the important error sources are usually come from feedback-resistor accuracy, temperature stability of circuit components, front-end input bias current, input voltage/current noise, Johnson thermal noise, offset voltage errors and bandwidth/stability limitations. Based on QuTRA publication and results measurement uncertainty better than µΩ/Ω was achievable with modest budget and off the shelf room-temperature electronics. Since I often perform in-house calibrations for my resistors, I am naturally curious to try this novel amplifier circuit and compare it’s performance to commercial DCC resistance bridges like MIL 6010B and MIL 6000A. Amplifier is only part of the story to perform a resistance measurement and I’ll need quiet and stable voltage source and high performance voltmeter. This shouldn’t be a problem since xDevs Lab has Fluke 5720A, zener standards and glorious HP 3458A for voltmeter role.
Original publication and design files
This project is based on implementation from this open-access publication. This impressive work was also presented at Conference on Precision Electromagnetic Measurements 2026 held in Madrid, Spain early September 2026.
Conceptually this amplifier can be used to realize a precise resistance comparator. With simplified connection diagram show below it is possible to compare known resistance standard R_STD to unknown R_DUT by measuring ratio between voltages at P1 Sense and P2 Sense terminals with high resolution voltmeter. Also because of expected very low bias current to QuTRA input it is possible to build a resistance comparator with only 3-terminal connections to each resistance standard in circuit. QuTRA converts current from the R_STD and generates inverted voltage thru R_DUT to maintain the middle node between resistors at virtual zero potential.
In theory such circuit could provide good performance with help of low-noise stable voltage source like Fluke 5720A and long-scale voltmeter like HP 3458A. In concept this simple circuit would work with any amplifier but in practical circuit input bias current, noise and gain stability of TRA would limit such measurement to 4-5 digits at best. This novel QuTRA design now with it’s composite design can push the uncertainty to µΩ/Ω region. Thankfully xDevs Lab has both calibrators, voltmeters, stable known resistance standards and automated resistance bridges that can be used to verify and characterize results of QuTRA approach and design in practice.
xDevs circuit variant implementation, Rev 1.2
I’ve decided to try my own implementation with schematics almost identical to original author’s concept. For this I’ve developed a small four-layer PCB tailored to fit into cast aluminum enclosure for shielding. Artwork of each layer is available here. To test the design here PCBway was used to manufacture bare boards. Design is open-hardware just like the original publication release to help promote new metrology tools and instrumentation for public experiments. Rendering of the PCB is here:
Top side:
This circuit is very sensitive to leakage and to mitigate this I’ve chosen Rogers RO4350B material for this build. Rogers materials often can be found in RF and high-frequency designs but here it is used for different set of properties. RO4350B has two magnitudes higher surface and volume resistivity compared to high quality TG170 fiberglass-epoxy dielectric materials and much better dielectric strength. Thermal conductivity and mechanical properties of Rogers materials are also superior to regular dielectrics which can be useful to maintain stable operating conditions of the circuit under changing environmental conditions. These improved properties come at a price and full Rogers-based PCB costs at least 3-5 times more than TG170 FR4.
xDevs QuTRA PCB has 4 copper layers, with outline dimensions 114 × 60 mm. Total stackup thickness is 1.6 mm. PCB surface has immersion silver finish with blue soldermask and yellow silkscreen. Copper thickness for all layers chosen to 1 oz (35 µm). Otherwise board specifications and fabrication parameters are rather typical for 4-layer stackup. Bottom rendering side:
Milled slots are added around some critical components to further isolate and reduce leakage areas. This board would require baby care and gloves during assembly and handling to avoid additional contamination of the surfaces that would degrade the performance of amplifier.
Hardware assembly and parts
All components were selected and ordered from well known component supplier, to minimize risk of reconditioned or faulty components parts in this project. PCB was manufactured by PCBway using their standard process for Rogers RO4350B material at 4 layers and 1.6 mm stack up. Reason for Rogers dielectric for this board is to minimize leakage and environmental conditions influence to the sensitive current inputs of this circuit. Board was manufactured and delivered after 3 weeks, including shipping transit. Total parts list including PCB and metal shielded box for this project ended up around 400 USD. Original opamps from published circuit were substituted with following parts:
- TI LMP7721 replaced by Texas Instruments OPA928DR or Analog Devices ADA4530-1.
- TI OPA627AU replaced with OPA828IDR
- Analog LT6016 replaced with TI OPA2182
- Analog LT3094 replaced with Analog LT3090
Here’s the BOM list.
| Item | Manufacturer | Reference | Component value | Part Number | QTY | Price (DK) OCT.2026 | Total | Notes |
|---|---|---|---|---|---|---|---|---|
| 1 | KEMET | C1 | C1206C102G2GECAUTO | CAP CER 1206 1NF 200V C0G 2% | 1 | 1.77 | 1.77 | 1nF C0G |
| 2 | KEMET | C2 | C1206C680J5GACTU | CAP CER 68PF 50V C0G/NP0 1206 | 1 | 0.5 | 0.5 | 68pF C0G |
| 3 | KEMET | C23,C25,C27,C29,C31,C33,C35,C37,C39, C41,C60,C69,C70,C71 |
T495×476K035ATE300 | CAP TANT 47UF 10% 35V 2917 | 14 | 2.521 | 35.294 | 47uF |
| 4 | KEMET | C24,C26,C28,C30,C32,C34,C36,C38,C40,C42,C72 | C1206C104M3GECTU | CAP CER 1206 0.1UF 25V C0G 20% | 11 | 0.623 | 6.853 | 100nF C0G |
| 5 | KEMET | C3,C4,C5,C6,C7,C8,C9,C10,C11,C12, C13,C14,C15,C16,C17,C18,C19,C20,C21,C22 |
C1206C103J2GACTU | CAP CER 10000PF 200V C0GNP0 1206 | 20 | 0.3206 | 6.412 | 10nF C0G |
| 6 | TDK | C43 | C3216×7R1H225K160AB | CAP CER 2.2UF 50V X7R 1206 | 1 | 0.454 | 0.454 | 2.2uF |
| 7 | TDK | C44,C46,C63,C64,C68 | CGA3E2×8R1E104K080AE | CAP CER 0.1UF 25V X8R 0603 | 5 | 0.129 | 0.645 | |
| 8 | TDK | C61,C66,C75 | CGA5L1×7R1E106K160DC | AUTOMOTIVE GRADE,1206,X7R,25V,10 | 3 | 0.437 | 1.311 | |
| 9 | TDK | C45,C62,C65,C67,C74,C76 | C1608×7R1H474K080AC | CAP CER 0.47UF 50V X7R 0603 | 6 | 0.24 | 1.44 | |
| 10 | onsemi | CR1,CR2,CR3,CR4 | LL4148 | DIODE STANDARD 100V 200MA SOD80 | 4 | 0.075 | 0.3 | |
| 11 | Kingbright | D10,D11 | APT3216LZGCK | Green 525nm LED Indication – Discrete 2.65V 1206 | 1 | 0.407 | 0.407 | |
| 12 | Littelfuse Inc. | D5,D6 | SMAJ16A | TVS DIODE 20VWM 32.4VC DO214AC | 2 | 0.48 | 0.96 | |
| 13 | Murata | FA1,FA2,FA3,FA4 | NFE61PT681B1H9L | FILTER LC(T) 680PF SMD | 4 | 0.534 | 2.136 | |
| 14 | Samtec Inc. | J4 | HTSW-104-07-G-S | CONN HEADER VERT 4POS 2.54MM | 1 | 0.296 | 0.296 | Power input |
| 15 | TE Connectivity Linx | J5,J6,J7 | SMA_FEMALE | CONN SMA RCPT STR 50 OHM PCB | 3 | 2.94 | 8.82 | Analog ports |
| 16 | Samtec Inc. | J8 | HTSW-102-07-G-S | CONN HEADER VERT 2POS 2.54MM | 1 | 0.157 | 0.157 | Range select |
| 17 | Samtec Inc. | J9 | HTSW-103-07-G-S | CONN HEADER VERT 3POS 2.54MM | 1 | 0.229 | 0.229 | H VLIM (1-2) jumper |
| 18 | Rose Enclosures | M1 | 10813060 | Box Metal, Aluminum Gray (125 mm x 80 mm X 57 mm) | 1 | 50.71 | 50.71 | DIY box |
| 19 | Panasonic Industry | R10 | ERA-8ARB912V | RES SMD 9.1K OHM 0.1% 1/4W 1206 | 1 | 0.98 | 0.98 | |
| 20 | Panasonic Industry | R82 | ERA-3AEB202V | RES SMD 2K OHM 0.1% 1/10W 0603 | 1 | 0.077 | 0.077 | |
| 21 | Panasonic Industry | R81 | ERA-3AED102V | RES SMD 1K OHM 0.5% 1/10W 0603 | 1 | 0.067 | 0.067 | |
| 22 | Panasonic Industry | R80 | ERA-3AED4992V | RES SMD 49.9KOHM 0.5% 1/10W 0603 | 1 | 0.067 | 0.067 | |
| 23 | Panasonic Industry | R11 | ERA-8AEB53R6V | RES SMD 53.6 OHM 0.1% 1/4W 1206 | 1 | 0.27 | 0.27 | |
| 24 | Panasonic Industry | R17 | ERA-8APB223V | RES SMD 22K OHM 0.1% 1/4W 1206 | 1 | 0.729 | 0.729 | |
| 25 | Panasonic Industry | R5,R13,R19 | ERA-8APB303V | RES SMD 30K OHM 0.1% 1/4W 1206 | 3 | 0.688 | 2.064 | |
| 26 | Panasonic Industry | R14,R18 | ERA-8APB473V | RES SMD 47K OHM 0.1% 1/4W 1206 | 2 | 0.688 | 1.376 | |
| 27 | Panasonic Industry | R6,R12,R15 | ERA-8ARB103V | RES SMD 10K OHM 0.1% 1/4W 1206 | 3 | 0.816 | 2.448 | |
| 28 | Panasonic Industry | R3,R4,R9 | ERA-8ARW102V | RES SMD 1K OHM 0.05% 1/4W 1206 | 3 | 1.111 | 3.333 | |
| 28 | Panasonic Industry | R16 | ERA-8ARW2491V | RES SMD 2.49KOHM 0.05% 1/4W 1206 | 1 | 1.34 | 1.34 | |
| 29 | Vishay | R79 | MCT0603MZ0000ZP500 | RES 0 OHM JUMPER 1/10W 0603 | 1 | 0.09 | 0.09 | |
| 30 | Vishay | R30,R84,R85 | MCU08050Z0000ZP500 | RES 0 OHM JUMPER 1/8W 0805 | 3 | 0.168 | 0.504 | |
| 31 | Susumu | R31 | RG1608P-154-B-T5 | RES SMD 150K OHM 0.1% 1/10W 0603 | 1 | 0.1 | 0.1 | |
| 32 | Susumu | R32,R33 | RG1608P-472-B-T5 | RES SMD 4.7K OHM 0.1% 1/10W 0603 | 2 | 0.1 | 0.2 | |
| 33 | Stackpole Electronics Inc | R7 | RNCF1206TKY49K9 | RES 49.9K OHM 0.01% 1/4W 1206 | 1 | 1.438 | 1.438 | |
| 34 | TE Connectivity | R8 | RQ73C2B150KBTD | RES 150 KOHMS 0.1% 0.5W 1206 | 1 | 0.634 | 0.634 | |
| 35 | Susumu | R83 | RG1608P-304-D-T5 | RES SMD 300K OHM 0.5% 1/10W 0603 | 1 | 0.1 | 0.1 | |
| 36 | Texas Instruments | U1 | OPA928DR | IC OP AMP 1CH 8-SOIC femtoampere bias current | 1 | 36.71 | 36.71 | Alt ADA4530-1 |
| 37 | Analog Devices | U10 | LT3045EDD#TRPBF | LDO IC Positive Adjustable 1 Output 500mA 10-DFN (3×3) | 1 | 10 | 10 | |
| 38 | Analog Devices | U11 | LT3090EDD#PBF | LDO IC Negative Adjustable 1 Output 600mA 10-DFN (3×3) | 1 | 7.9 | 7.9 | |
| 39 | Texas Instruments | U2,U3,U4 | OPA828IDR | J-FET Amplifier 1 Circuit 8-SOIC | 3 | 8.71 | 26.13 | |
| 40 | Texas Instruments | U5,U6 | OPA2192IDR | Standard (General Purpose) Amplifier 2 Circuit Rail-to-Rail 8-SOIC | 2 | 5.92 | 11.84 | |
| PCBway | PCB1 | W08139BSJ148 | xDevs QuTRA Rev1.2 PCB, 4L, RO4350B, Blue, ImmAg | 1 | 78 | 78 | Custom PCB | |
| VPG | R40 | 10 | Optional test resistors, VPG VHP10x/VHP20x | 1 | 0 | Option | ||
| VPG | R41 | 10k | Optional test resistors, VPG VHP10x/VHP20x | 1 | 0 | Option | ||
| VPG | R42 | 10k | Optional test resistors, VPG VHP10x/VHP20x | 1 | 0 | Option | ||
| VPG | R43 | 10K/5.1K | Optional test resistors, VPG VHD | 1 | 0 | Option | ||
| 305.1 USD | Digikey Price | |||||||
| Option for lower voltage +/-8V with ADA4530-1 | ||||||||
| 31 | Susumu | R31 | RR0816P-7872-D-87C | RES SMD 78.7KOHM 0.5% 1/16W 0603 | 1 | 0.1 | 0.1 | Alt for ADA4530-1 |
| 35 | Susumu | R83 | RG1608P-154-B-T5 | RES SMD 150K OHM 0.1% 1/10W 0603 | 1 | 0.1 | 0.1 | Alt for ADA4530-1 |
| 36 | Analog Devices | U1 | ADA4530-1ARZ | IC OP AMP 1CH 8-SOIC femtoampere bias current | 1 | 41.95 | 41.95 | Alt for ADA4530-1 |
| Cleaning flux remover for PCB assembly | ||||||||
| Techspray | 1634-12S | G3 NO-CLEAN FLUX REMOVER G3 | 1 | 36 | 36 | |||
Optional resistors R40,R41,R42,R43 are placed on the PCB and connected to form a divider for validation and testing purposes and not a part of the actual amplifier itself. These are just helper footprints for experiments and tests of the amplifier noise floor in fully shielded environment without inevitable external noise sources. Bill of materials is also available in PDF and Excel formats:
QuTRA Bill of Materials to order, Excel
QuTRA Bill of Materials to order, PDF
Option available for using different input amplifier, such as Analog Devices ADA4530-1ARZ. This amplifier is pin-to-pin compatible to primary TI OPA928 but has lower maximum operating voltage rating. This is easy to incorporate in the same QuTRA PCB by replacing R31 and R83 resistors that define output voltage of positive and negative voltage regulators. If you opt for ADA4530-1 populate these alternative resistors instead of original listed in main BOM. I have also decided to try Texas Instruments OPA828IDR instead of expensive TI OPA627AU/2K5 for secondary stages.
And schematics PDF export of the design here as well:
QuTRA schematics Revision 1.2 based on 10.1109/TIM.2026.3711328
Second page shows linear low-noise regulator to improve PSRR of amplifier. It is designed to accept any bipolar input voltage in range from 9 to 20 VDC. Both positive and negative supplies are mandatory for amplifier to operate properly.
Assembly process and photographs
PCBway manufactured 10 blank boards and they were received on October 29, 2026. They were ordered on September 30, 2026. Total amount paid for this was 1159 CAD, including customs and shipping cost. Since this project is fully open-source you can order your own copy of boards by using PCBway shared project link. This is not sponsored project and boards were paid by author in full.
I’d suggest placing and soldering sensitive parts like U1 amplifier, SMA connectors and C3-C22 capacitor network by hand to avoid contamination of the board surface by flux residues. In either case wash PCB with strong cleaning solution to get rid of any contaminants and bake it after assembly. Handle the board by edges only and wear gloves to avoid any oils on the circuit surfaces.
I’ve opted for immersion silver plating on the open pads to avoid thick nickel plating that is used in ENIG process to protect gold from copper diffusion. Inputs and output have footprint for SMA thru-hole connector but you could just solder wires if board is used in the shielded enclosure.
All components are placed on top side only, bottom side is only used for non-sensitive routing, ground and power planes and guarded metal layer around input port.
To help with assembly I’ve also ordered frameless stencil for paste mask top layer. This can be used to print solder paste on the pads and assembly board in reflow oven.
Now to assembly…
Power-up and initial checks
Power bring-up
Amplifier bring-up
Test currents
Output stability
Operational margins
Power consumption
Initial benchmarks and validation
Assembled circuit prototype was built and ready for testing. For reference instrumentation I’ll use following equipment:
- Fluke SL935 10000 Ω standard with known history
- ESI SR104 10000 Ω standard with known history
- xDevs.com RWS106 1 MΩ transfer standard, based on VPG VHA518-11 element
- xDevs.com RWS107 10 MΩ transfer standard, based on Caddock USF resistive element
- Fluke 5720A 7½-digit calibrator as voltage source
- Fluke 732B 10 V DC voltage standard as alternate voltage source
- HP 3458A 8½-digit voltmeter
- Keithley 6430 Femtoampere SourceMeter
- MIL 6010B DCC resistance bridge
- MIL 6000A BVD high-resistance bridge system
Practical application and results
Comparison of 1 kΩ:10 kΩ resistance standards
Comparison of 10 kΩ:10 kΩ resistance standards
Comparison of 1 MΩ:1 MΩ resistance standards
Comparison of 100 MΩ:100 MΩ resistance standards
Conversion of input currents to output voltage
Conclusion and summary
In this article we’ve designed and investigated performance of interesting composite trans-resistance amplifier and demonstrated it’s performance by set of experiments with help of resistance reference standards and various DUT units. Performance of QuTRA replicated by xDevs.com was confirmed good and I’m looking forward to seeing what other metrology members can do with it. Few prototypes were assembled on October 30, 2026 and will be used for testing various sources and resistors in future. I’m pretty happy with the results and hopefully will see more progress on QuTRA as well. Hopefully this publication and others will inspire today’s generation of analog design engineers and metrologists to join this very interesting (and often challenging) field of industry.
Author would like to credit authors Marta Musso, Massimo Ortolano and Michele Perotti of the original QuTRA for making their project open-hardware, Todd M. and Igor O. for valuable support for this project all heroes in metrology community who inspired me and made stuff like this possible by sharing their own projects. Further discussion is very welcome at our own IRC xDevs.com chat server: irc.xdevs.com (port 4808, channel: #xDevs.com) or in xDevs.com Forum. If you have information about your own TRA/TIA amplifiers not mentioned or listed in this article, feel free to provide them and we can showcase them on site too.
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Modified: Oct. 29, 2026, 5:02 a.m.