| Electronics > Projects, Designs, and Technical Stuff |
| how to choose an active oscilloscope probe for 0-2v antenna measurements? |
| << < (4/4) |
| David Hess:
I found a couple more examples of this kind of circuit worth studying in the LT1010 datasheet. Instead of using two JFETs with one correcting the offset of the other, a low input bias current operational amplifier is used to correct the offset of the JFET. As shown below, a chopper stabilized operational amplifier is used but any low input bias current part would work like a bipolar LT1008/LT1012/LT1097 or a JFET/CMOS part like a TL071. The trade off might be worth it to avoid the need for a matched JFET or trimmer. The disadvantage of this form of offset correction and why it is not used in oscilloscopes at least in this form is that if the input is overloaded, the operational amplifier's slow control loop causes very slow recovery time but this is probably not a consideration in your application. |
| k8943:
Really interesting, thanks. The LT1010 / LT1050 combo explodes the BOM cost in a way noticeable in a throwaway in-circuit probe. Suggests building it standalone. Looked at datasheets but not sure how to figure out if would retain precision running at a voltage spread greater than the +5v/-5v cited. JFETs arrived but PCB stuck in China until after the end of their New Year's celebrations in a couple of weeks. Looking forward to testing - still struggle to believe the PNP/NPN pairs 1v:1v. |
| David Hess:
--- Quote from: k8943 on February 01, 2019, 03:55:50 pm ---The LT1010 / LT1050 combo explodes the BOM cost in a way noticeable in a throwaway in-circuit probe. Suggests building it standalone. Looked at datasheets but not sure how to figure out if would retain precision running at a voltage spread greater than the +5v/-5v cited. --- End quote --- I meant that as an example to study and not a literal implementation. Replace the LT1010 with the cheap class-A 2N3904/2N3906 buffer in Bob Pease's design which I showed first and use your favorite JFET or superbeta operational amplifier like the LT1012/LT6010 in place of the LT1050. |
| k8943:
Finally got to testing the probe built into NFC board. Whilst the attachment of a passive probe at the measurement point seems to reduce the reading on the active circuit by about 5%, the readings from both approaches vary markedly. (Image attached - CH1 Pease, CH2 standard probe connected upstream - note long timebase.) Very worthwhile! Thanks for this and look forward to taking it further as iterate the board/antenna and fine tune the Rx circuit. |
| Navigation |
| Message Index |
| Previous page |