Design of MX LTZ1000A DC voltage reference

Contents

Intro

MX magic write-up goes here..

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Project goal and specifications

  • Maximum stability for DC voltage reference
  • Low tempco precision wire-wound resistors, PCB for both PWW Ultrohm Plus and Vishay VPG resistors.
  • Linear LTZ1000 ultrazener [4]
  • Thermal sensor NTC for external calibration purpose
  • Compact 80 × 120 mm form factor to fix Hammond metal enclosure
  • 1ppm/year voltage output
  • 7.15VDC ±1% direct output
  • Thermostat setpoint +55°C
  • Kelvin sense output
  • Single-rail voltage input
  • 2.54mm SIP connection ports
  • PCB space for add-on board to use direct output from LTZ1000
  • Basic reverse supply voltage protection.
  • Room for 3D printed cap on top and bottom of board for LTZ
  • Room for 3D printed cover for rest of board.
Item Value Units
Input voltage Vin, reference module +18.0 VDC
Output current, Reference output at VREF port 1 mADC
Operating temperature range 0 to +50 °C
Storage temperature range 0 to +80 °C
Lead temperature (soldering, 10s) +240 °C


Schematics 1: Linear LTZ1000 reference design schematics (old-rev)

Now to stay on safe side, base reference circuit will repeat datasheet schematics with very minor changes.

Test ports definitions are as shown in tables below

Board reference Schematic signal name Direction,type Description
J1.1 VCC Input, power Positive power input (+9 to +24VDC)
J1.2 VCC Input, power Positive power input (+9 to +24VDC)
J1.3 GND Input, power Power return
J1.4 GND Input, power Power return

Table 3: Port definition J1

And output of the reference available at J2 port.

Board reference Schematic signal name Direction,type Description
J2.1 VREF+ Output, sense U1 LTZ1000 pin 3 sense output
J2.2 VZENER Output, drive Zener reference drive output
J2.3 RETURN Output, drive Signal ground return output
J2.4 VREF- Output, sense U1 LTZ1000 pin 7 sense output

Table 4: Port definition J2

Board reference Schematic signal name Direction,type Description
J3.1 +12V Power, +12V Polarity protected power output
J3.2 GND Power, GND Power ground return

Table 4: Port definition J2

Board reference Schematic signal name Direction,type Description
J4.1 NTC-A Output, NTC NTC resistor pin 1
J4.2 NTC-B Output, NTC NTC resistor pin 2

Table 4: Port definition J2

Layout design


Image 3: xDevs.com MX voltage reference (Rev.A00) PCB cross-section


Image 3: xDevs.com MX voltage reference (Rev.A00) PCB cross-section

Assembly notes

Total BOM list of components used is listed in table.

Digikey Part Number Manufacturer Manufacturer Part Number Customer Ref Number Customer Description Quantity for module
N/A Ultrohm Plus N/A R1-R5 Custom made wirewound set 1
495-2479-1-ND EPCOS (TDK) B32529C1104J289 C1,C2,C4 0.1uF Cap Film 5% 100VDC 3
495-2489-1-ND EPCOS (TDK) B32529C1223K289 C3,C6,C7 22nF Cap Film 5% 100VDC 3
399-3905-1-ND KEMET T495×476K035ATE300 C5,C8 47uF Cap Tant. 10% 35V 2
1N4148FS-ND Fairchild/ON Semiconductor 1N4148 D1-D3 1N4148 Gen Purp DO35 3
PZT3904CT-ND Fairchild/ON Semiconductor PZT3904 Q1 Transistor NPN 1
1.00KXBK-ND Yageo MFR-25FBF52-1K R6 1k ohm 1/4w 1% 1
10.0KXBK-ND Yageo MFR-25FBF52-10K R7 10k 1/4w 1% 1
1.00MXBK-ND Yageo MFR-25FBF52-1M R8 1M 1
402KXBK-ND Yageo MFR-25FBF52-402K R9 402k ohm 1/4w 1% 1
10.0XBK-ND Yageo MFR-25FBF52-10R R10 10 ohm 1/4w 1% 1
BC2299-ND Vishay BC Components NTCLE203E3103FB0 R11 Thermistor 10k 1
LTZ1000CH#PBF-ND Linear Technology LTZ1000CH#PBF U1 Shunt Voltage Reference 1
LT1013ACN8#PBF-ND Linear Technology LT1013ACN8#PBF U2 IC OpAmp 1
HM1023-ND Hammond Manufacturing 1457K1202BK N/A Enclosure, Black 1
HM1022-ND Hammond Manufacturing 1457K1202 N/A Enclosure, Natural 1
1457K1202EBK-ND Hammond Manufacturing 1457K1202EBK N/A Enclosure, Black EMI Shielded 1
1457K1202E-ND Hammond Manufacturing 1457K1202E N/A Enclosure, Natural EMI Shielded 1

Table 7: MX Ref A00 BOM-list with estimated cost per each part

Linear parts were purchased directly from their website shop, Bulk Metal Foil resistors were ordered from Vishay Precision Group, rest of parts come from Digikey USA.

Assembly small passive components, such as capacitors, SMT resistors first. Install diodes and power transistor Q1. Then you can install temperature sensor ICs U3 and U4. You can test temperature sensor operation by connecting power supply to J5 port and measuring temperature outputs at J4.


Image 3: xDevs.com MX voltage reference (Rev.A00) PCB cross-section


Image 3: xDevs.com MX voltage reference (Rev.A00) PCB cross-section


Image 3: xDevs.com MX voltage reference (Rev.A00) PCB cross-section


Image 3: xDevs.com MX voltage reference (Rev.A00) PCB cross-section


Image 3: xDevs.com MX voltage reference (Rev.A00) PCB cross-section

MX Voltage reference module designed for education and hobby experimentation purposes only, which resulted in used components choices. If one need ultra-stable voltage reference for production device or practical equipment, very different approach would be required. This includes adding necessary polarity/overload/ESD protection circuits to the input and outputs of the reference, etc.

To reduce thermal stress to foil resistors, heatsink legs with small pliers or braid during soldering, so resistor body would not heat up as much. Excessive heat on resistive element will cause stress and possible hysteresis, which will require long-term recovery to original state afterwards. Make your soldering brief and short. If solder joint not good, wait till board and resistor cool down back to ambient and then try brief reflow for bad connection again. Do not keep iron tip heating up resistor lead longer than few seconds.

Initial data results

Connect power to the modules from quiet (electric noise wise) linear power supply, leave to settle for few days/weeks/months and then measure with best stability multimeter available.

Make sure power supply polarity is correct before power on.
Reverse polarity even for brief seconds can reliably destroy expensive LTZ1000 chip.

Here are some measurements and test data completed on 5pcs LTZ1000 modules:

Test result description Datalog
Initial, 7.096052 V

Table 8: xDevs.com MX reference module test log

If you interested in this project build or have any questions, feel free to jump in comments. We also have more detailed and less traditional KX LTZ1000-reference build so be sure to check that one out as well.

Author: Todd M.
Published: July 23, 2017, 3:39 p.m.
Modified: Aug. 1, 2019, 9:15 a.m.

References

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