Keithley_DMM7510 burden voltage

[brumbarchris]

I am just looking in the datasheet of the Keithley DMM7510 (attached only the two relevant pages) and in the table on page 10 a 200mV burden voltage is indicated for the 100mA range. This would indicate a shunt resistance of some 2 Ohm. However, in the last table on page 11, an Effective Internal Sunt Value for the same range is indicated as 0.1Ohm. I am not sure I understand the discrepancy, can somebody explain?

Best regards,
Cristian

[Kleinstein]

The first 2 tables with the higher drops seem to include the drop on the fuse and switching part. This is especially obvious for the higher currents. Normally one has to keep the drop at the shunt rather small at something like 3 or 10 A to limit self heating.

The last table to a large part looks like the actual shunt resistor used. However the value of 0.1 Ohms still looks low for the 100 mA range. 1 Ohms would make more sense  - so maybe an additional misprint there.

The low current ranges(<=10 mA)  seem to use a trans-impedance amplifier and thus way get so low voltage drops.

[tooki]

For what it’s worth, in my 6.5 digit Keithley 2015, it uses a 0.1 Ohm shunt for the 100mA range, with a max burden voltage of 30mV, according to the specs in the manual.

So the 7510 spec doesn’t make obvious sense to me, either.

[Kleinstein]

The Keithley 2015 like the 2000 is more like a low cost version with few shunts. So they use the same shunt for different ranges, compromising performance. So the 100 mA range would a kind of scaled up 1 A range.

The DMM7510 is a higher grade instrument, and likely uses separate shunts, expect for likely 1 A and 3 A. The specs suggest a separate shunt for the 100 mA range.

[HighVoltage]

I just did a quick test on my DMM7510


mA range
1.1 mA, 11 mV, ~ 10 Ohm
10 mA, 12.7 mV, ~ 1.2 Ohm
100 mA, 126 mV ~ 1.2 Ohm

A range
0.2 A, 64 mV ~ 0.3 Ohm
1A, 0.3 V ~ 0.3 Ohm

So, at least 3 different shunt resistors.

[Kleinstein]

The specs for the 1 mA and 10 mA are a bit better than the higher ranges. I doubt they could get this with just 10 mV burden on a simple shunt. So I am quite sure the lower current ranges use trans-impedance amplification instead. It gets obvious from the very last table in the datasheet part in the PDF file:  only 150 nA drift without AZ would not work well with a 1 Ohms shunt - so more like an TIA with some 100 Ohms in the feedback.  I am still not sure why they have the variable resistance in the other lower ranges.

[Brad O]

I am just looking in the datasheet of the Keithley DMM7510 (attached only the two relevant pages) and in the table on page 10 a 200mV burden voltage is indicated for the 100mA range. This would indicate a shunt resistance of some 2 Ohm. However, in the last table on page 11, an Effective Internal Sunt Value for the same range is indicated as 0.1Ohm. I am not sure I understand the discrepancy, can somebody explain?

The difference comes from the path resistance.  The sense resistor itself is 0.1 ohms but there are switches and other hardware between the signal and that sense resistor that add to the burden voltage. 

[The Soulman]

I am just looking in the datasheet of the Keithley DMM7510 (attached only the two relevant pages) and in the table on page 10 a 200mV burden voltage is indicated for the 100mA range. This would indicate a shunt resistance of some 2 Ohm. However, in the last table on page 11, an Effective Internal Sunt Value for the same range is indicated as 0.1Ohm. I am not sure I understand the discrepancy, can somebody explain?

The difference comes from the path resistance.  The sense resistor itself is 0.1 ohms but there are switches and other hardware between the signal and that sense resistor that add to the burden voltage.

Also in that path is a high voltage fast acting fuse, these expensive fuses actually have much more resistance than a normal fuse.