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MSO8104A - Measurement Accuracy (SUBT, ADD)

Question asked by cwosnitza on Aug 27, 2007
Latest reply on Sep 11, 2007 by marowley

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Hallo,
I have some questions according to the accuracy of the measurement functions:

1. What is the accuracy for a subtraction of two channels using
":FUNCtion<N>:SUBTract <operand>,<operand>" command:
a) dc Voltage Measurement Accuracy, Dual Markers = Â±[(dc gain accuracy)+ (resolution)]
b) dc Voltage Measurement Accuracy, Single Marker = Â±[(dc gain accuracy) + (offset accuracy) + (resolution/2)]

2. What is the accuracy for the sum of two channels using
":FUNCtion<N>:ADD <operand>,<operand>" command:
a) dc Voltage Measurement Accuracy, Dual Markers = Â±[(dc gain accuracy)+ (resolution)]
b) dc Voltage Measurement Accuracy, Single Marker = Â±[(dc gain accuracy) + (offset accuracy) + (resolution/2)]

The Programmer's Reference gives no information about the accuracy of the implemented functions and measurements at all and the description in the data sheet is not clearly understandable.

Regards,
Christoph

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dnt Aug 28, 2007 1:32 PM

See the programmer's reference manual on the :wav:data? query.
It describes the how the data is coerced from the 8bit ADC to final
form(ASCII,WORD,BYTE). The primary component in accuracy of the Math functions will be the DC gain of the ADCs.

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cwosnitza Sep 3, 2007 2:12 AM

In my understanding each channal has his own offset calculation/setting mechanism, isn't it?
In this case the offset accuracy of each input channal of the math function has to be considered and the accurancy of the result is equal to:
Â±2*[(dc gain accuracy) + (offset accuracy) + (resolution/2)]

Is it correct or do I see this wrong?

Many regards,
Christoph

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dnt Sep 5, 2007 3:14 PM

Each channel has a calibration setting but I don't think this is germane
to your calculation. The calibration setting is there to give linearity
to the ADC through a specific range. It is not linear in itself so each point
in the ADC can have a different calibration offset.

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cwosnitza Sep 6, 2007 6:07 AM

Scope help:
â€œThe purpose of offset whether it is in the probe or the front end stage of a scope channel is to allow the subtraction the dc component of the input waveform. Subtracting the dc component allows for the better use of the dynamic range of the input.â€

If it is possible to subtract the dc component of a channel, than it is indispensable to generate the dc component. Each generator has a specific accuracy. If two channels are used for a calculation, then two dc generators with two different accuracies are involved to the calculation. In this case the offset settings and offset accuracies of both channels are germane to the calculation and to the measurement uncertainty analysis, isnâ€™t it?

Christoph

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marowley Sep 11, 2007 2:14 PM

Letâ€™s setup a situation to measure:

Measure a 100mv DC signal on Channel one and add it to channel two that has a 200mV DC input.

Channel 1 is set to 20mv/div
Channel 2 is set to 50mV/div.

We will use a Vavg measurement which uses the dual cursors more accurate and easier for me doing the calculation.

Channel One
dc Voltage Measurement Accuracy Â±[(dc gain accuracy)+ (resolution)]
dc gain accuracy = +1.25% of Full scale (20mV *8)+ resolution (.4% full scale)
1.25% of 160mV = 2mV + .4% of 160mV= 640uV

dc gain accuracy= Â±2.64mV of Channel one

Channel Two
dc Voltage Measurement Accuracy Â±[(dc gain accuracy)+ (resolution)]
dc gain accuracy = +1.25% of Full scale (50mV *8)+ resolution (.4% full scale)
1.25% of 400mV = 5mV + .4% of 400mV= 1.6mV

dc gain accuracy= Â±6.6mV of Channel Two

Function to add Chan 1 and Chan 2 worse case if the errors went in the same direction = 9.24mV
Function to subtract Chan1 from Chan 2 worse case if the errors went in the opposite direction = 9.24mV

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5 Replies

dnt Aug 28, 2007 1:32 PM
Mark Correct
Correct Answer
See the programmer's reference manual on the :wav:data? query.
It describes the how the data is coerced from the 8bit ADC to final
form(ASCII,WORD,BYTE). The primary component in accuracy of the Math functions will be the DC gain of the ADCs.
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cwosnitza Sep 3, 2007 2:12 AM
Mark Correct
Correct Answer
In my understanding each channal has his own offset calculation/setting mechanism, isn't it?
In this case the offset accuracy of each input channal of the math function has to be considered and the accurancy of the result is equal to:
Â±2*[(dc gain accuracy) + (offset accuracy) + (resolution/2)]

Is it correct or do I see this wrong?

Many regards,
Christoph
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dnt Sep 5, 2007 3:14 PM
Mark Correct
Correct Answer
Each channel has a calibration setting but I don't think this is germane
to your calculation. The calibration setting is there to give linearity
to the ADC through a specific range. It is not linear in itself so each point
in the ADC can have a different calibration offset.
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cwosnitza Sep 6, 2007 6:07 AM
Mark Correct
Correct Answer
Scope help:
â€œThe purpose of offset whether it is in the probe or the front end stage of a scope channel is to allow the subtraction the dc component of the input waveform. Subtracting the dc component allows for the better use of the dynamic range of the input.â€

If it is possible to subtract the dc component of a channel, than it is indispensable to generate the dc component. Each generator has a specific accuracy. If two channels are used for a calculation, then two dc generators with two different accuracies are involved to the calculation. In this case the offset settings and offset accuracies of both channels are germane to the calculation and to the measurement uncertainty analysis, isnâ€™t it?

Christoph
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marowley Sep 11, 2007 2:14 PM
Mark Correct
Correct Answer
Letâ€™s setup a situation to measure:

Measure a 100mv DC signal on Channel one and add it to channel two that has a 200mV DC input.

Channel 1 is set to 20mv/div
Channel 2 is set to 50mV/div.

We will use a Vavg measurement which uses the dual cursors more accurate and easier for me doing the calculation.

Channel One
dc Voltage Measurement Accuracy Â±[(dc gain accuracy)+ (resolution)]
dc gain accuracy = +1.25% of Full scale (20mV *8)+ resolution (.4% full scale)
1.25% of 160mV = 2mV + .4% of 160mV= 640uV

dc gain accuracy= Â±2.64mV of Channel one

Channel Two
dc Voltage Measurement Accuracy Â±[(dc gain accuracy)+ (resolution)]
dc gain accuracy = +1.25% of Full scale (50mV *8)+ resolution (.4% full scale)
1.25% of 400mV = 5mV + .4% of 400mV= 1.6mV

dc gain accuracy= Â±6.6mV of Channel Two

Function to add Chan 1 and Chan 2 worse case if the errors went in the same direction = 9.24mV
Function to subtract Chan1 from Chan 2 worse case if the errors went in the opposite direction = 9.24mV
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MSO8104A - Measurement Accuracy (SUBT, ADD)

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