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NEW TOPIC: 7818 Offset Capability Uses: WAS 7A18 option1?

 

Hi Gudjon,

The 7A18 Option 6 (DC Offset) is not a differential amplifier plugin. Under a few very limited circumstances you can use it to combine two signals differentially. The limitations are important. It is easy to draw erroneous conclusions about what you are seeing when you do not understand the limitations or you are not aware that you have exceeded the limitations of the 7A18 opt 6 circuit design.

A Tektronix differential amplifier plugin requires a much more advanced design. There are two 7000 series differential amplifiers: The 7A13 and the 7A22. Both the 7A13 and the 7A22 have extremely large dynamic ranges and the ability to sum two signals that are offset by a large voltage.

A Differential Amplifier is a precision circuit that takes the difference between two signals. In order to be useful this kind of amplifier needs to have a very wide dynamic range in other words I may be required to measure the difference of a few microvolts (7A22) in two signals, or the difference between signals of 100V (7A13) that only differ by a few millivolts or less. It may be required to measure the voltage between two 100MHz signals (7A13) so both inputs and signal paths must have identical phase shifts over frequency. Sometimes it is necessary to accurately measure the DC offset level of an AC signal (7A13) such as when you are troubleshooting a switching mode power supply. The 7A22 is a differential amplifier designed for a different purpose. It has its own unique features such as the ability to set the LF and HF cutoff points to reduce noise. It measures down to microvolts and in order to this you need to reduce the bandwidth and have very low noise amplifiers.

The 7A18 Offset capability lets you see a signal riding on a large DC level. The dynamic range above and below that DC level may not be great. Just because there are two channels and they both have offset capability that does not necessary mean if you invert channel 2 and set the Display Mode to ADD you will get the same result the 7A13 or 7A22 because the dynamic range of the circuit that does the ADD may not have a very wide dynamic range. (You have to look at the manual to see what the dynamic range is)

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Gudjon Gudjonsson
Sent: Thursday, September 12, 2019 12:25 PM
To: TekScopes <TekScopes@groups.io>
Subject: Re: [TekScopes] 7A18 option1?

Hi list

On Thu, Sep 12, 2019 at 8:13 PM Dennis Tillman W7PF <@Dennis_Tillman_W7PF> wrote:
Option 6 added the very useful Offset capability to the plugin.
Dennis Tillman W7PF
Dennis. Can you please explain a bit more for what measurements this feature is useful?
How does it compare with the 7a13 plugin?
(Sorry if I am hijacking the thread)

Regards
Gudjon




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Dennis Tillman W7PF
TekScopes Moderator