The dynamic impedance of the 399 is around 1ohm, of the 1399 0.011ohm to 0.04ohm.
To divert a "significant current" off the 399 the input impedance of [...]
At 1 mA output current, what is the required precision? If you use wiring with e.g. 0.1 Ohm, you already get an error of about -14 ppm = 1 mA * 0.1 Ohm / 7 V). You probably need a 4-wire system with two buffer amplifiers, one for plus and one for minus output.
At 1 mA output current, what is the required precision? If you use wiring with e.g. 0.1 Ohm, you already get an error of about -14 ppm = 1 mA * 0.1 Ohm / 7 V). You probably need a 4-wire system with two buffer amplifiers, one for plus and one for minus output.
hi Dieter,
you raise a good point that i'd not thought about - given just 1mA of current flow through a set of leads having 0.1 ohm of resistance will cause a drop of 100uV across said leads. this is a good enough reason to always keep the current drawn from any voltage reference to an absolute minimum.
but the BF245C FET can still serve to protect the output of the ICL7650S. is this useful? or would it be preferable to use the opamp output pin directly as the reference voltage output.
cheers,
rob :-)
There needs to be a resistor between opamp output and gate. Also the output should get some protection against overvoltage or reverse voltage (from external sources like ESD).
but of course you can also use a LT1001 as Op-Amp if you regard the offset as part of the zener voltage.
Especially if you have not so much temperature change in your lab as I have in mine (18-34 deg C over the year)
would adding a JFET to the output of the opamp (but within the feedback loop) offer any disadvantage? this would also eliminate the potential of any internal heating of the opamp die caused by the output load.
At 1 mA output current, what is the required precision? If you use wiring with e.g. 0.1 Ohm, you already get an error of about -14 ppm = 1 mA * 0.1 Ohm / 7 V). You probably need a 4-wire system with two buffer amplifiers, one for plus and one for minus output.
hi Dieter,
you raise a good point that i'd not thought about - given just 1mA of current flow through a set of leads having 0.1 ohm of resistance will cause a drop of 100uV across said leads. this is a good enough reason to always keep the current drawn from any voltage reference to an absolute minimum.
A weak point with the circuit as shown may be the reaction to capacitive loading of the output (e.g. a cable or DMM internal capacitance and the 11 V zener). With too much capacitance (no so clear where the limit is, maybe 1 nF or so) the buffer could oscillate. With less capacitance there could still be a voltage shift as it changes the impedance need by the AZ amplifier input. I would add the local feedback capacitor (~ 1 nF) and resistor (e.g. 1 K) in the feedback path to make the buffer more resitant to to capacitive loading.
David: so you believe that using a nulled second LT1001 will provide similar performance to an ICL7650S+JFET? i had shied away from using an LT1001 because of the untrimmed maximum input offset voltage of 60uV; if this LT1001 is considered a 'sacrificial buffer' then changing it out for a new one could introduce a significant shift. but if this offset can be reliably nulled out that is not an issue.
I have noticed in other's reference designs that offset nulling is generally not used, but then this could be because most folks are targeting a 10v output rather than just a zero-gain buffer, and that with a gain stage the offset is just a constant that can be ignored.
R+C added to feedback path:
The capacitor should be from negative input to directly output of the OP-Amp. Otherwise you pick up too much EMI from the output line. I would also add a 100nF (foil) capacitor in parallel to the output against EMI.
I hope the 24V for the heater are stabilized (the LM399 is sensitive to heater voltage variations).
PS: couple of weeks back I started a separate thread here on nulling the opamp's offset and its TC. Still not sure how to exactly do it in this wiring, moreover I would be happy to read more on why the TC of the input offset gets zero when its offset is nulled.. On the other hand doing the nulling with a crappy 10T trimmer is not a good idea either (so you have to replace the trimmer with some quality resistors at the end). What had been told there - there is a chance to compensate the TC of the 399 - and that sounds pretty interesting..
BTW., why do you mess with two opamps now? You went from the 7V version towards the 10V version and that is a standard version people use here since [for]ever, with a single opamp and the transistor(s)/fet buffer/limiter, its schematics depicted almost once a week here..
..
The subject is covered in old application notes from about the time that the OP-07 was released. Linear Technology may have published something about it also.
I know there was a more detailed publication, but check out PMI (Precision Monolithics Inc.) application note AN-12 which covers the math of what is going on with a matched differential pair..
Andreas, is the below what you had in mind?
i was hoping to get away with not needing to closely regulate the heater voltage,
b) its time that you read the LM399 thread most of your questions are handled there:
https://www.eevblog.com/forum/metrology/lm399-based-10-v-reference/msg441913/#msg441913
I hope the 24V for the heater are stabilized. the LM399 is sensitive to heater voltage variations.
i presume that each run was conducted over a relatively short time interval?