Resistor divider for differencial probe - Page 1
Forum section: electronics / projects

author: bernroth
author_rank: Regular Contributor
timestamp: June 30, 2014, 07:57:38 am
content: Hello!In order to measure ripple voltage of DC/DC converters (among other applications) I was purchasing recently an Agilent N2818A high voltage (+-20V) differential probe. Previously I was thinking about purchasing another probe with higher input voltage capability to be able to measure SMPSs. Unfortunately these probes do not have the required sensitivity at very low voltage levels.Now, instead of buying two probes I'd like to discuss the idea of making a resistor divider box to make the N2818A probe capable to measure switch mode power supplies (up to +-500V)I have attached a first sketch of how the schematic could look like.The probe itself has a bandwidth of 200MHz. When using the resistor divider box I do not expect to get full bandwidth, rather only a small portion of it (which will be okay anyhow).What do you think?Regards,Bernhard

author: mij59
author_rank: Frequent Contributor
timestamp: June 30, 2014, 09:17:11 am
content: Hi,Please follow the link, its a divider for the HP 1141a, should work for other probes as well.https://www.eevblog.com/forum/reviews/agilent-(hp)-1141a-differential-probe-and-power-module-1142a/

author: bernroth
author_rank: Regular Contributor
timestamp: June 30, 2014, 09:54:17 am
content: mij59: Thank you for the link.I updated my schematic (attached picture).In comparison to the schematic in the other article I added two 1M2 resistors instead of one 2M4 and three varistors to protect the sensible inputs of the differencial probe.I understand the purpose of the trimmers C4 and C4 but it is not yet obvious to me the purpose of R10.What are typical values for C1, C2, C3, C4 and R10?

author: mij59
author_rank: Frequent Contributor
timestamp: June 30, 2014, 10:49:32 am
content: Hi,I don't think the varistors will work because of leakage current.Don't know what happens with the probe if you exceed the maximum ratings.For dc and low frequencies the divider ratio is determent by R7, R1, R3, and a part of R10 AND the input resistance of the probe to ground, check the data sheet of the probe.For high frequency the divider ratio is determent by C1, C3 AND the input capacitance of the probe to ground, also parasitic capacitance has to be accounted for.R10 is labeled LF CMRR witch stands for low frequency common mode rejection ratio, if the input voltage +in equals _in, adjust R10 so that the voltage at the +probe equals -probe.Sorry don't have typical values for C1, C2, C3, C4 and R10.

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 30, 2014, 12:05:50 pm
content: coincidentally i'm waiting my parcel for the 100X and AC attenuator for my 1141 (sadly i still dont have 10X in the set), whats more interesting is how they do it in the so much expensive attenuator. someone sell each one for $300 used, i got those 2 for around $100 similar to when i get my 1141 + 1142, quickly grab it and wait. hope this thread still alive when the things arrived for me to post some picture.making matched resistor+caps divider is difficult esp when we dont have proper resources and guide from experienced. you may be interested at 100X probe but looking back at the 1141 manual the attenuator + 1141 can only do +-30V differential which is sad since last time i want full differential of +-500V, lets see what it has in there.

author: Andreas
author_rank: Super Contributor
timestamp: July 01, 2014, 02:29:45 am
content: Quote from: mij59 on June 30, 2014, 10:49:32 amthey also have a large (voltage dependant) capacitance -> they will not work at all at higher frequencies.The capacitors have to be inversely to the Resistor ratio. (together with the input capacitance of the scope input).With best regardsAndreas

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2014, 02:59:06 am
content: One of those polymer breakdown TVSs would work... like uh... dang, can't seem to find them. Well, there's these,http://www.digikey.com/product-detail/en/PESD0603-240/PESD0603-240CT-ND/1813513but why don't they provide a V-I curve?Downside is, they seem to act like either a GDT (doesn't turn on until ~100V) or have notable ESL or something to that effect (compare: junction diode forward recovery), hence letting through a few nanoseconds of a relatively high spike in an IEC 61000-4-2 test.Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 20, 2014, 12:00:38 pm
content: keysight/agilent/hp 2145 100x attenuator (voltage divider) pcb this is how they do it... the inputs can take ±500 V(dc + peak ac) easy...