Help me understand BODE PLOT. - Page 1
Forum section: electronics / beginners

author: Cacao
author_rank: Contributor
timestamp: June 28, 2017, 07:16:09 am
content: I have this simulation of a difference amplifier IC, INA149, on LTspice. I already tried googling it but I still cannot understand bode plot. Is the output correct? Thank you for the help.Here's the plot and schem:https://ibb.co/dUt03Q

author: Rerouter
author_rank: Super Contributor
timestamp: June 28, 2017, 08:04:07 am
content: The top line is telling you how much the signal is attenuated (reduced in amplitude) vs frequency,The bottom line is telling you how much the output signal is delayed vs the frequency,

author: danadak
author_rank: Super Contributor
timestamp: June 28, 2017, 10:38:47 am
content: Google "understanding bode plots", lots of good info, video out there.Regards, Dana.

author: rstofer
author_rank: Super Contributor
timestamp: June 28, 2017, 06:39:19 pm
content: Given that the INA149 has a -3 db point at about 300 kHz, it's not clear to me why your circuit is -3 db down at 20 Hz. I could be missing something...Figure 13 here:http://www.ti.com/lit/ds/symlink/ina149.pdfI don't understand the units of the right hand Y axis and I don't see V+in or V-in on the plot although they are listed. Are the two AC supplies in phase with each other (net input of 0V)?I don't have a model for the INA149 so I can't try to duplicate your setup.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 28, 2017, 06:57:54 pm
content: Looks like V+in and V-in are both on top at the 0dB point. Also looks like there is (almost) no phase shift between the 2 inputs, resulting in almost no output. (-106dB?)

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: June 28, 2017, 07:15:24 pm
content: don't even try .AC until after you see correct operation in .TRAN and you don't have bad startup transients

author: Dave
author_rank: Super Contributor
timestamp: June 28, 2017, 07:15:51 pm
content: Quote from: rstofer on June 28, 2017, 06:39:19 pmThe dashed line in LTspice represents phase, not gain.I think op is trying to measure CMRR with his setup, but the SPICE model of the amplifier might not be detailed enough to render a usable result.

author: rstofer
author_rank: Super Contributor
timestamp: June 28, 2017, 09:36:43 pm
content: Quote from: Dave on June 28, 2017, 07:15:51 pmAh... I don't know why people accept the default black background. The red traces, if any, are also a problem on a black background. My old eyes didn't even notice the dash in the line.

author: Cacao
author_rank: Contributor
timestamp: July 02, 2017, 11:14:59 am
content: As far as I know the output must be 0dB because it has the same input but what I dont understand is that why the output is like that ( the phase line starts at -10db and the magnitude is at -105db). Is there something wrong with the set up?

author: Cacao
author_rank: Contributor
timestamp: July 02, 2017, 11:19:17 am
content: Quote from: Dave on June 28, 2017, 07:15:51 pmis it really because of the SPICE model that I am getting this output? or are there other things that I am wrong? Sorry Im kinda late on the reply

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 02, 2017, 11:22:55 am
content: If you feed the +IN and -IN with the same signal you will not have any output.

author: Damianos
author_rank: Frequent Contributor
timestamp: July 04, 2017, 07:36:09 pm
content: Quote from: Cacao on July 02, 2017, 11:14:59 amThe phase is not measured in dB, it is measured in degrees. So the phase starts at 0oand delays about 0.0065oat 1kHz.The amplitude, because the inputs are shorted, is the result of the offset (of the used model). That is about -105dB in relation of the input voltage.