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What is the amplifier bandwidth of 428?

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0lolol0
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What is the amplifier bandwidth of 428?

Post by 0lolol0 » March 21st, 2015, 4:46 am

Hello, I'm using the 428 now.

What is the amplifier bandwidth of 428? Is it as a function of rise time or gain? How could I set the value of amplifier bandwidth?

If i set the gain of 10to10, what would the amplifier bandwidth and filter frequency be?

Many thanks in advance.

brian.d.smith
Keithley Applications
Keithley Applications
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Re: What is the amplifier bandwidth of 428?

Post by brian.d.smith » March 22nd, 2015, 4:16 pm

Please download the 428 Instruction manual from our web site. Table 3-10 shows the low pass filter response for all of the filter settings. Appendix F shows response of the amplifier for various gain settings with the filter off.

brian.d.smith
Keithley Applications
Keithley Applications
Posts: 402
Joined: June 10th, 2013, 6:00 am
Country: United States

Re: What is the amplifier bandwidth of 428?

Post by brian.d.smith » March 23rd, 2015, 7:21 am

"Thanks for your reply.

I downloaded the 428 Instruction Manual Rev. D, but no information of amplifier bandwidth for the 10to10 gain setting was found in Appendix F. Could you please provide some information on this issue?

Meanwhile, I found information from Brochure of 428 that, bandwidth = 0.35 / rise time. Does it mean the amplifier bandwidth depend on the filter setting (rise time)?

Thank you."


You are correct. The 10^10 setting does not have an associated plot. There are plots for 10^6, 10^8 and 10^9. If you carefully look at the -5db down point, it appears that for each higher decade there is about a 1/2 drop of bandwidth compared to the previous decade.

The 10^9 range -5db point looks like about 7kHz. I would expect the 10^10 range to be -5dB down at about 3.5 kHz.

The filter settings are in addition to the unfiltered response of the amplifier as per the instruction manual.

If you must know the exact response of the 10^10 range, you should measure the response of the 10^6, 10^8 and 10^9 ranges and make sure that you can re-create these response curves. That will prove your methodology and measuring equipment. An arbitrary function generator and an oscilloscope should be adequate to the task.

0lolol0
Posts: 6
Joined: November 29th, 2012, 5:22 am
Country: Indonesia

Re: What is the amplifier bandwidth of 428?

Post by 0lolol0 » March 28th, 2015, 11:37 pm

Hello. Dr.brian.d.smith

If the current of interest is several to hundreds of pA, what would be the appropriate gain that I can choose? Or several choices are available for me to choose depending on the different requriement on noise level, aren't they?

I carefully read the 428 manual, and got to know that the overall frequency response of the instrument is determinded by the gain setting and the selected rise time (Page 3-12). To be frank, such calculation or plotting (Fig 3-7 Typical combined response) beyonds me. If you do not really mind, could you please also provide the overall frequency response value corresponding to the gain and rise time you recommend, or the somewhat more detailed calculation method avaialbe to follow? Many thanks in advance.

brian.d.smith
Keithley Applications
Keithley Applications
Posts: 402
Joined: June 10th, 2013, 6:00 am
Country: United States

Re: What is the amplifier bandwidth of 428?

Post by brian.d.smith » April 1st, 2015, 6:18 am

Are you measuring a DC current or an AC current?

The model 428 is an obsolete product and I do not have one immediately available that I can try out to give you an opinion based on hands-on experience.

In addition, my guess is that your model 428 has not been calibrated in a very long time. Please correct me if I am wrong. If your model 428 has not been calibrated recently, then you should devise a way to perform a confidence check against one or more known good standards.

If you are measuring DC current, the lowest cost new instrument to use would be a Keithley 6485.

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