DESIGN IDEA
By J. Yannone
Rejustor-based Precision MilliOhm
Meter
1
Introduction
0
A high-precision low-cost milliOhm meter is described. This instrument can be used to measure
resistance in the range of 0.001 Ohms to 10 Ohms in 0.001 Ohm increments.
This laboratory grade instrument uses a calibrated voltage reference, voltage to current converter,
difference amplifier and a low cost microcontroller containing a 14 bit Sigma-Delta analog to digital
converter to accurately measure low resistances using 4-wire probes.
The milliOhm meter uses Rejustors to calibrate precision voltage references, convert voltage to
current and create an accurate difference amplifier with high common mode rejection.
2 Background
Precision milliOhm meters are used to measure low resistances, where a typical digital multimeter
is not accurate. These applications include measuring the resistance of switches, relays,
connectors and wire bonds, just to name a few.
MilliOhm meters are available commercially from vendors such as Keithly and Instek. However,
these tend to be expensive, particularly when compared with the precision and production cost of a
Rejustor-based solution.
Overall this is a simple design. A more complex design could be made using multiple resistance
networks to scale the current and gain of the difference amplifier in order to increase the range of
resistances that this device can measure.
Figure 1: MilliOhm Meter Block Diagram
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infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under
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3 Detailed description:
The circuit, shown in Figure 2 can be broken down into the following basic building blocks:
• Power Supply
• Precision Voltage Sources
• Voltage to Current Converter
• Difference Amplifier
• Microcontroller with 14-bit ADC
Figure 2: MilliOhm Meter Schematic
3.1 Power Supply
The Power Supply can be an off-the-shelf low noise +/-15VDC Power Source (i.e. a 200mA similar
to the one provided with the MBK-408A Rejustor Calibration Kit would be fine).
3.2 Precision Voltage Sources
The voltage source is a Linear Technologies LT1236A-10 ultra-low drift and noise precision
reference with excellent long-term stability. R59 is two halves of the Microbridge MBD-902-XL
wired in series to achieve an equivalent resistance close to 75K Ohm (9K + 63K = 72K Ohm with
no calibration). R57 and R58 are two halves of a Microbridge MBD-333-AL that are calibrated so
the output of the reference is as close to 10.000VDC as possible.
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Voltage follower U8B provides a buffered 10.000VDC reference voltage. Q1 is used to turn on and
off (microcontroller controlled) current to the resistance to be measured. R211 is a 100K Ohm
resistor used to limit the current when Q1 is on. A voltage divider fed from the 10.000VDC
reference and voltage follower (U8A) is used to produce the 1.6384VDC reference voltage for the
14-bit Delta Sigma ADC. The R213/R214 divider is the Microbridge MBD-103-CS Rejustor with 1:5
resistance ratios. R213/R214 are calibrated so that the top resistor is approximately 5.1035 times
the bottom resistor (R214 ~ 5.1035 x R215) until the output of U8A is 1.6384VDC.
The governing equation is:
Vout = Vin ×
By making Rtop ~ 5.1035 x Rbottom,
Rbottom
Rtop + Rbottom
Vout = 0.16384 × Vin
The calibration of these circuits is performed at room temperature, the intended operating
environment for the instrument, and starts by calibrating the 10V reference and progresses in turn
to the lower reference voltage.
3.3 Voltage to Current Converter
A low power voltage to current converter (U7B) is based on the Howland Current Pump. By making
R208=R205, and R209=R206+R207, the circuit will output a current governed by the following
equation:
Current output =
(R 206 + R 207) ×Vin
(R 207 × R 205)
Since the voltage input is 10VDC and by making R205 = R206+R207 and R207 = 1K , this
simplifies to:
I=
10VDC
= 10mA
1KΩ
this is the current that will be fed through the resistance to be measured.
Resistors R205, R206, R208 and R209 are Microbridge MBD-903-AS Rejustors. These
components are adjusted to a resistance of 85K , with exception of R206 trimmed to 84K . R207
(1 K ) could be a custom part sourced from Microbridge or a parallel combination of several larger
value Microbridge Rejustors. For the circuit to balance,
R 209 (R 206 + R 207)
=
R 208
R 205
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3.4 Difference Amplifier
U7A and resistors R201 through R204 form a gain of 10 difference amplifier.
If the ratio
R 204 R 202
=
, then this circuit simplifies to:
R 203 R 201
Vout =
R 202
× (Vsense _ high − Vsense _ low )
R 201
By making R202 = 10* R201, this becomes:
Vout = 10(Vsense _ high − Vsense _ low )
This is the voltage directly across the resistor to be measured. The sense leads (placed as close to
the resistance to be measured) are part of a 4-wire connection that ignores voltage drops in the
current carrying wires from the voltage to current converter and to ground.
This circuit uses one Microbridge MBD-103-AL Rejustor for R201 and R203 (8.5K) and one
Microbridge MBD-903-AS Rejustor for R202 and R204 (85K).
The use of the Rejustors calibrated to 0.01% allows us to manufacture a low cost precision
difference amplifier with common mode rejection ratio of better than 80dB.
R 202
R 201
4 × 0.001
1+
CMRR = 20 log
= 88.7dB
This high CMRR is only possible with tight matching that can be achieved through precision
calibration of Rejustors.
3.5 Microcontroller with 14-bit ADC
The software and full implementation of the microcontroller and user interface are beyond the
scope of this paper. It can be assumed that the necessary talent required to implement this portion
of the design is readily available for outsourcing.
A likely candidate for the microcontroller would be the Texas Instruments MSP-430. The
microcontroller will allow the user to indicate when a reading should be made and then report the
calculated result to the user via an LCD display or over an RS-232 interface with a PC.
The resistance, based on Ohm’s law, is simply the voltage measured by the sense leads divided
by 10mA, which is the known current supplied by the voltage to current converter.
Rmeasured =
(Vsense _ high − Vsense _ low )
10mA
Since the gain of the difference amplifier is 10,
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Output _ of _ the _ difference _ amplifier
Rmeasured =
10mA
10
Or, Rmeasured = 10 x voltage measured by ADC
The reference voltage supplied to the microcontroller is 1.6384V, so that every ADC bit represents
100 microvolts.
1.6384Vdc 0.001Vdc
=
bit
16,384
The following table shows the expected voltages for various resistance values:
Table 1: Measured Resistance versus ADC Count
As can be seen, 1 milliOhm (0.001 Ohm) represents 1 bit (0.0001 V), so the output count from the
ADC is equal to the resistance being measured in milliOhms.
4 Instructions for Use
Place measurement probes across low value resistance to be measured and press the measure
button. Read resistance value from LCD display.
Alternatively, send command from PC (LabVIEW application or HyperTerminal) to the instrument
and read result on the PC monitor.
5 Benefits of Using Rejustors
Rejustors are used exclusively in this design to take the place of fixed resistors, laser trimmed
resistors and/or trimmer potentiometers.
By taking the appropriate amount of time to calibrate (this high precision instrument can tolerate
longer calibration to improve results) 0.01% precision can be obtained.
Some companies do make 0.01% resistors, but they are very expensive and usually have limited
values selection (examples include 0.01% metal foil resistors manufactured by Riedon and 0.01%
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wire wound resistors manufactured by Huntington Electric). Calibrating the Rejustors at room
temperature and using the instrument in a room temperature environment will only improve the
already impressive stability of these low TCR (temperature coefficient of resistance) devices.
The Rejustors are more cost effective than mechanical trimmer potentiometers and digital
potentiometers (which usually don’t have very good resolution). Also, the ability to automate the
calibration of the Rejustors is both less time consuming and less error prone than if a technician
had to manually adjust trim potentiometers.
Another disadvantage over mechanical trimmers is that Rejustors are immune to vibration and
operate over a wide temperature range.
The ability to software calibrate the Rejustor is also a far superior alternative to binning precision
resistors (lots of time associated with measuring, collecting and hand picking resistors).
Ratio calibration and availability of in circuit settable precision resistors is where the Rejustor
excels.
6 Performance
An example of a commercially available milliOhm meter is the Instek GOM 802 available from
Instek (http://www.instek.com/GOM-802.htm) for $510. This instrument has an accuracy of 1% of
the reading for low resistances (< 30 mOhm). Keithly Instruments Inc. also makes equipment with
better specifications that cost thousands of dollars.
The specifications for this precision low cost milliOhm meter include:
• Full-scale output range of 1 milliOhm to 10 Ohms
• Output resolution is 1 milliOhm
• Accuracy expected to be better than +/-5 milliOhm (not tested)
Using Rejustors allows us to make a device that will have similar performance at a fraction of the
cost.
7 Future Work
Overall this is a simple design. A more complex design could be made using multiple resistance
networks to scale the current and gain of the difference amplifier in order to increase the range of
resistances that this device can measure.
8 Test results
Although this circuit has not been tested, the author has tested difference amplifier circuits used
with 10-bit A/D converters to measure resistances in the 500 m to 10 range with better than
±5m accuracy. The circuit tested only used 0.1% resistors.
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