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For the InfiniiMax probes, what effect does the "Signal being probed" setting have? Why do I have to specify single-ended or differential?

Infiniium scopes and InfiniiMax differential probe amplifiers have a unique method of applying dc offset. This method considers whether the signal being probed is single-ended or differential. It allows the full benefits of differential probing of single-ended waveforms without sacrificing offset range. Understanding how to properly use offset for your application ensures that you get the maximum performance and dynamic range from your InfiniiMax probes.

When using an InfiniiMax probe, you can specify whether the signal being probed is single-ended or differential in the Infiniium scope's Probe Setup dialog box. To bring up this dialog box, click Setup | Channel n and then the Probes button. The "Signal being probe" setting is important, as it controls how the scope adds offset when you rotate the channel's offset knob.

InfiniiMaxProbe Screenshot

Regardless of how offset is applied, its purpose is to allow the subtraction of the dc component of the input waveform. Subtracting the dc component lets you use the dynamic range of either the probe or scope more efficiently. The following is a simplified drawing of how the offset voltage is applied for InfiniiMax probes.

InfiniiMax Probe Offset Voltage

Depending on the type of waveform you are probing and the probe head you are using, the scope applies the offset voltage differently. There are 3 different measurement configurations:

  • Single-ended probe head probing a single-ended waveform
  • Differential probe head probing a single-ended waveform
  • Differential probe head probing a differential waveform

Single-ended probe head probing a single-ended waveform

When you use one of the single-ended probe heads, you can only measure a single-ended waveform. The Infiniium scope sets its internal offset to zero and adds the offset before the input to the differential amplifier. Therefore the signal’s offset will not use any of the amplifier's dynamic range. Since the offset is subtracted before the differential amplifier, the available offset range is greater than when probing a differential signal.

The offset range and dynamic range for the InfiniiMax probe amplifiers is as follows.

Probe amplifierDynamic rangeOffset range (single-ended)
113xA±2.5 V±12.0 V
1168A, 1169A3.3 V peak-to-peak±16.0 V

Note: The minus input of the probe is unused when using single-ended probe heads.This is normal and causes no problems.

Differential probe head probing a single-ended waveform

When you use one of the differential probe heads and select a single-ended waveform, you can make higher bandwidth and more accurate measurements than with the single-ended probe head. But the scope applies offset the same way it does with a single-ended head; the scope sets its internal offset to zero and adds the offset before the input to the differential amplifier. The signal's offset will not use any of the amplifier’s dynamic range and the available offset range is greater than when probing a differential signal.

The offset range and dynamic range for the InfiniiMax probe amplifiers is as follows.

Probe amplifierDynamic RangeOffset range (single-ended)
113xA±2.5 V±12.0 V
1168A, 1169A3.3 V peak-to-peak±16.0 V

Differential probe head probing a differential waveform

When you use one of the differential probe heads and select a differential waveform, the Infiniium scope sets the probe's internal offset to zero. Typically, the plus and minus sides of the differential input signal have the same dc component. This dc value is removed by the differential amplifier and the output waveform is centered about ground. The common mode range of the probe determines the maximum allowable dc component. The common mode range is ±6.75 V for all InfiniiMax probe amplifiers.

When you rotate the channel's offset knob, the scope adds the offset voltage internally. The particular model of Infiniium scope you have determines the available offset range. See below for the offset range of your scope.

Input ZVertical sensitivityAvailable offset
DSO8xxx4A offset range  
50Ω1 mV to ≥ 25 mV/div±0.4 V
 > 25 mV to ≥ 50 mV/div±0.9 V
 > 50 mV to ≥ 90 mV/div±1.6 V
 > 90 mV to ≥ 200 mV/div±3.0 V
 > 200 mV±4.0 V
5485xA offset range:  
50 ΩAll±12 division or ±4 V,
whichever is smallest
5485xB/D offset range:  
1 MΩ1 mV to < 10 mV±2 V
 10 mV to < 20 mV±5 V
 20 mV to < 100 mV±10 mV
 100mV to < 1 V±20 V
 1 V to 5 V±100 V
50 Ω1 MV to 5 mV±2 V
 5 mV to < 200 mV±5 V
 200 mV to 1 V±20 V
5484xA/B offset range:  
1 MΩ2 mV to 104 mV/div±4 V
 > 104 mV to 2 V/div±40 V
50 ΩAll±12 divisions
5481xA/5482xA/54835A offset range:  
50 Ω, 1 MΩ1 mV to 49 mV/div±2 V
 > 49 mV to 249 mV/div±10 V
 > 249 mV to 1.24 V/div±50 V
 > 1.24 V to 5 V/div±250 V