Buffer for Kelvin Varley Divider? - Page 1
Forum section: electronics / testgear

author: VintageNut
author_rank: Frequent Contributor
timestamp: June 07, 2015, 10:03:08 pm
content: I have a General Resistance KV divider of 7 decades. I would rather take readings directly than use a bridged configuration.The ideal output buffer would be an electrometer grade op amp configured as a non-inverting buffer. The problem is that these buffers seem to have 10 microvolt per degree temperature change.Has anyone implemented such a buffer successfully for use with a KVD?

author: Vgkid
author_rank: Super Contributor
timestamp: June 07, 2015, 11:14:25 pm
content: Why, most meters have a 10Gohm input impedance. Wont matter much.

author: VintageNut
author_rank: Frequent Contributor
timestamp: June 07, 2015, 11:28:59 pm
content: The output impedance of the KVD is measured at about 25k ohms max. Using a 10G input impedance this creates at 2.5 ppm error. The error is dependent on the KVD setting.The KVD has 0.1 ppm resolution. Using a DMM without a buffer wipes out the 7th decade and two counts of the 6th decade.The error can be observed using Fluke 332A.

author: Marco
author_rank: Super Contributor
timestamp: June 08, 2015, 01:14:45 am
content: AD8638? Auto-zero, so negligible tempco for offset, with below pA typical bias current.

author: VintageNut
author_rank: Frequent Contributor
timestamp: June 08, 2015, 05:24:44 pm
content: Quote from: Marco on June 08, 2015, 01:14:45 amThank you for the suggestion. My reading of the datasheet revealsTypical input bias current 1.5 pA, maximum 40 pA. Not low enoughInput voltage range -0.1V to +3V Not practical as a bufferPlease let me know if I missed something.

author: edavid
author_rank: Super Contributor
timestamp: June 08, 2015, 05:44:43 pm
content: Jim Williams used an LTC1152 and LT1010 to achieve 0.4ppm error:http://cds.linear.com/docs/en/design-note/dsol11.pdf

author: VintageNut
author_rank: Frequent Contributor
timestamp: June 08, 2015, 05:52:08 pm
content: Quote from: edavid on June 08, 2015, 05:44:43 pmPerfect! Thanks.

author: VintageNut
author_rank: Frequent Contributor
timestamp: June 08, 2015, 06:06:42 pm
content: Quote from: edavid on June 08, 2015, 05:44:43 pmThe datasheet for the LTC1152says that input bias current is +/- 10 pA typical and +/- 100pA max. I wonder if he ever evaluated the effect of the LTC1150 input bias on his KVD?

author: edavid
author_rank: Super Contributor
timestamp: June 08, 2015, 07:13:34 pm
content: Quote from: VintageNut on June 08, 2015, 06:06:42 pmIt's right there in his error budget.

author: VintageNut
author_rank: Frequent Contributor
timestamp: June 08, 2015, 09:43:12 pm
content: Quote from: edavid on June 08, 2015, 07:13:34 pmOk. I see it. Thanks for pointing it out. His KVD must be 10K ohms output to have 0.1 ppm error at 10 pA bias. That would be 0.25 ppm for my KVD. I guess obtaining some samples and measuring will be in order. It will be a good use of the 236 SMU.

author: Marco
author_rank: Super Contributor
timestamp: June 08, 2015, 09:48:24 pm
content: Quote from: VintageNut on June 08, 2015, 05:24:44 pmYou're not making electronics for an automotive environment here ... specs over temperature range are irrelevant.QuoteYou read the specs for 5V single ended supply, not the 16V supply (also you should feed it ~-2 to +14 to get better performance around 0).The relevant bias current information for you is figure 14 page 7 (0.1 pA typical at 25 degrees Celsius). Worst case is irrelevant, test the IC and if it doesn't perform well enough try another till you get a typical one.

author: splin
author_rank: Frequent Contributor
timestamp: June 08, 2015, 10:55:33 pm
content: Quote from: edavid on June 08, 2015, 05:44:43 pmWonder why he chose the LTC1152 rather than the LTC1052C? Also interesting that the current datasheet specs are considerably worse than listed in that app note:LTC1052CLTC1152LTC1152 from Design Solution 11Bias current (typ) @ 25C1pA10pABias current (max) @ 25C30pA100pA50pABias current (max) over Temp175pA1000pAVos (typ)0.5uV1uVVos (max)5uV10uV5uVVos drift (typ)10nV/C10nV/CVos drift (max)50nV/C100nV/C50nV0-10Hz Vnoise (typ)1.5uV p-p2uV p-p (0.1-10Hz)CMRR (min)120dB115dB110dBCMRR (typ)140dB130dBAvol (min)120dB110dB140dBAvol (typ)150dB130dB Moved misplaced 50nV entry

author: VintageNut
author_rank: Frequent Contributor
timestamp: June 09, 2015, 04:14:48 am
content: Quote from: Marco on June 08, 2015, 09:48:24 pmWhich datasheet has input bias of 0.1 pA typical?

author: Marco
author_rank: Super Contributor
timestamp: June 09, 2015, 10:22:49 am
content: http://www.analog.com/media/en/technical-documentation/data-sheets/AD8638_8639.pdfpage 7 figure 14