Re: [volt-nuts] Matched resistors
On 07/17/2014 10:26 AM, Randy Evans wrote: Frank, The high cost is my concern, although high performance demands high price typically. I am trying to double the voltage reference from either an LM399 or LTZ1000, hence the need for precision matched resistors for a x2 non-inverting amplifier (using a LT1151 precision op amp). An alternative I am investigating is using the LTC1043 in a voltage doubling circuit as shown in Linear Technology app note AN 42, page 6, Figure 16. It states that Vout = 2xVin +/- 5 ppm. I am less concerned about the absolute accuracy than I am about the long term stability. I assume that a high quality capacitor is required (low leakage, low ESR, low dielectric absorbtion, etc.) but the circuit does not appear to be dependent on the absolute value of the capacitors. I'm not sure if the two 1uF caps need to be matched. If they do then that would be a show stopper. Does anyone have any experience using the LTC1043 in such a circuit? Hi Randy, There are some other error sources that might need to be considered when using the LTC1043. I have not used the LTC1043, but note that on the data sheet there is a small charge injection at each of the switch pins. In the multiply by 2 circuit shown on the data sheet they are using 1 ufd caps. Typical charge injection (depends on voltage level) is 8 pC. With the 1 ufd caps this is 8 uV. I assume there is some offsetting effect - but this might be a significant contributor to the 5 ppm error that is mentioned. There is also a 6 nA (typical) leakage mentioned. During the hold time ( about 1 msec) of the output 1 ufd cap this comes to 6 uV. Regards, Bob Smither attachment: smither.vcf___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
[volt-nuts] Data Precision 3600
Does anyone know the basic specs for a Data Precision 3600 - resolution, accuracy? Thanks! -- Bob Smither, PhD Circuit Concepts, Inc. = If you don't read the newspaper you are uninformed; if you do read the newspaper you are misinformed. --Mark Twain = smit...@c-c-i.com http://www.C-C-I.Com 281-331-2744(office) -4616(fax) attachment: smither.vcf___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Fluke 510A
On 08/10/2013 04:57 PM, Marv @ Home wrote: A 6.5 or better will find a good home and be appreciated in your lab, Joe. Many orphans need homes always appear on eBay, particularly the top line of its day HPs: 3455a, 3456a, and 3457a. Agreed! I bought a 3455A when it was new. It was a workhorse in my lab for years, then it sat on the shelf for a few years. When I fired it up a tantalum cap smoked. Replacing that had it running again. After a few months, a RAM chip went out, after replacing it another error surfaced that I have not tracked down. Since fighting the 3455A, I have purchased three 3457As on Ebay. They are always on and I have had no problems with them. They are my current favorite DMMs. All three indicate within a few ppm at +10.0 volts (I have not adjusted / calibrated them). I think HP got a lot right with the 3457A. Wish I could afford a 3458A! -- Bob Smither, PhD Circuit Concepts, Inc. = Government is not healthy for children and other living things. -- Jeff Daiell = smit...@c-c-i.com http://www.C-C-I.Com 281-331-2744(office) -4616(fax) attachment: smither.vcf___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] (no subject)
On 07/28/2013 04:56 PM, Tom Miller wrote: Can you get a service/calibration manual for it? I just sent my HP3457A out for calibration. $150 Hi Tom, May I ask where you had this done? It seems like a reasonable charge. What documentation did they return with the unit? Thanks, (another 3457A owner (actually three)) -- Bob Smither, PhD Circuit Concepts, Inc. = Blessed is he who expects no gratitude, for he shall not be disappointed. -- W. C. Bennett = smit...@c-c-i.com http://www.C-C-I.Com 281-331-2744(office) -4616(fax) attachment: smither.vcf___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] 731A output impedance
On 11/27/2012 07:37 PM, ed breya wrote: I'm not sure how much elaboration is needed, but here's some: If you take all of the feedback from the output terminal, that's better for DC accuracy by eliminating the voltage drop of the series resistor, while still providing some overload protection to the opamp. But, it also decreases phase margin so that it will be more prone to oscillate with capacitive load. If the series R becomes zero, the voltage drop and the extra loss of phase margin are eliminated, but the inability to drive large capacitive loads remains - it is a limitation of the amplifier. Usually a small amount of series R can help a lot with capacitive loading stability, but even when small it can drop enough DCV to be a problem. A common way to solve both problems is to sense the DC right at the output to eliminate the drop in the series R as above, but to increase stability by taking some AC ahead of the resistor - usually at the output of the amplifier. Figure 9 of the TI data sheet shows exactly what you are suggesting. If the amplifier has an integrating feedback capacitor, it's usually already connected that way, so only the resistive part of the feedback needs to go to the terminal. If there is no feedback capacitance, then a small amount can be added from the amplifier output to the effective inverting input. I don't know what the output stage of the 731A looks like, but it must be an inverting (integrator) amplifier or a buffer, if using an opamp. In either case there should be a way to modify the feedback network. However, whatever is changed or added may affect the overall frequency response and noise. Excellent points Ed. The output stage is a non-inverting amplifier with a small gain (about 1.3). The compensation of the lm301A is OK but I think it could be improved to better tolerate load capacitance. I have not looked at what would be required to move the op-amp sense point to the 731A output and leave the 1K inside the loop. I would prefer to not butcher the board. Clearly shorting the 1K is pretty easy! The lm301 is protected against shorts to ground. Thanks to all who added to this thread. -- = Bob Smither, PhD Circuit Concepts, Inc. I've come to realize that protecting freedom of choice in our everyday lives is essential to maintaining a healthy civil society. -- George McGovern smit...@c-c-i.comhttp://www.C-C-I.Com281-331-2744 = attachment: smither.vcf___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
[volt-nuts] 731A output impedance
The 731A is specified to have an output impedance of less than 1.1K in the 10V setting. A meter with 10 M input Z therefore causes a .01% (100 ppm) reading error. I understand that when operated using a null meter the output impedance is not a problem. I would like to use my 731A as a bench standard for my DMMs. The 10V output is taken through a 1K resistor (R18) from the output pin of the op-amp. Clearly the op-amp output pin is much lower impedance. The op-amp is an LM301A and appears to be conservatively compensated. I am thinking of shorting R18. I understand it provides some protection to the op-amp and may be a factor in the op-amp stability. Other than the above, can anyone see why this would be a bad idea? Thanks, -- Bob Smither smit...@c-c-i.com Circuit Concepts, Inc. 281-331-2744 attachment: smither.vcf___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Traveling Standards
On 10/18/2012 02:05 PM, Andreas Jahn wrote: Hello Bob, whats the matter with you. Infected by precision virus like me? You wanted to have a standard with about 10ppm and now you blame a 3-4ppm drift. :-) - afraid so Andreas! The LM199A is hermetically sealed. The PCB, the 8K Resistor and the voltage Regulator are not. Although a small sample, the two references appear to be similarly affected by whatever caused the drift - similar range of drift, similar time constant. On the first view I would blame it on the meter. It is very unusual that the drift of 2 different references has nearly exact the same amount of ppm and direction. But on the other side you state that there are several HP3458A which recorded the drift. It is not probable that all came freshly from calibration of a other location. Any ideas about what could cause the drift we are seeing? From time constant it could be the humidity change. This is my best guess. My 2 LT1027CCN8-5 references which ara mounted only with 1 Pin to the PCB have time constants in the range of 4-7 days. The epoxy material of a PCB should lie in the same ball park area. The LT1027 are influenced by around 0.5 ppm per percent humidity change. For the hermetically sealed brand new references LT1236AILS they state in their new product catalog a humidity change of less than 10ppm for 25% humidity change. (page 36) http://cds.linear.com/docs/Product%20Info/NPC.pdf This is most interesting - so even hermetically sealed units are influenced by humidity! I asked them whether this is from mechanical stress from the PCB and they confirmed to me that with a dead bug mounting the influence of humidity will be virtually unmeasurable. So they will delete the parameter from the data sheet. Mine are not dead bug mounted. The 'PCB' is in fact a Radio Shack perf board - certainly not the best substrate to mount them on - I don't think it is FR-4 material. So for the LM399 it might be mechanical stress introduced by the PCB. When looking at your cirquit there are several points to mention: One common failure source will be the LM78L15. A output voltage change will influence the supply of MAX6350 and the reference current of LM399. PSRR of MAX6350 is about 2-5 ppm/V above 10V supply. (without self heating effects). LM78L15 spec is 1mV/C. This would result in .02 ppm/C on the LM199A (operated at 1 mA with 8K resistor providing the current). For the MAX6350 1mV/C and 5 ppm/V = .005 ppm/C The LM399 resistor will give a current change of about 10% per Volt (100uA) resulting with 0.5 Ohm impedance in about 50uV/V or 7ppm/V see note above. Other weak points of the cirquit are: The LM399 heater voltage is not stabilized. this will give about 0.5ppm/V It is stabilized by the power supply - a 24 volt, .02%/C unit = 5mV/C = .0025 ppm/C. And finally: was the LM399 always in the same orientation during measurements? (will be difficult with a cylindrical housing). My LM399 drift 3-4 ppm by tilting orientation. Not sure - but from the consistent results (consistent drift and apparent settling) it likely was. The above still leaves humidity induced PCB changes causing mechanical stress as likely. I am going to look for some better board material for when I construct TS2. Thanks Andreas! -- As we act, let us not become the evil that we deplore. attachment: smither.vcf___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Traveling Standards
On 09/22/2012 02:24 PM, Bob Smither wrote: I just dropped off the Volt-Nuts TS at my shipper, headed to the fourth volunteer for this project. I will report his measurements as they become available. Some drift data provided by the fourth volunteer is now available here: http://c-c-i.com/ts -- Of all tyrannies, a tyranny exercised for the good of its victims may be the most oppressive. … [T]hose who torment us for our own good will torment us without end, for they do so with the approval of their own conscience. -- C.S. Lewis attachment: smither.vcf___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
