Electronics > Metrology

Making an array of LTZ1000

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dietert1:
Having six LTZ1000 in the drawer i studied a little how to use them as an array in an external oven. From my ADR1399 experiments i learned how to make ovens big enough for large filter caps and for 7 to 10 V gain stages.
A known way to adjust the TC of a LTZ1000 reference is a small resistor in series with the zener. It forms a voltage divider together with the 120 Ohm resistor that determines zener current. In effect we amplify the compensation of the temperature dependent Ube. The two resistors need to be precision parts and we can tune the gain a little using reasonable quality tuning resistors. To reduce number of iterations one needs to adopt a numerical model.
Using parts from the drawer i made two cells of the array and adjusted the first reference to zero TC using a hairdryer on the bench. The adjustment resistor of 18,33 Ohm consists of 2x UPW25 10R + MF 1% 220R. The second LTZ1000 got the same compensation and the board went into the incubator for temperature sweeps.
In the diagram readings are shown as deviation from the average voltage during the sweep:
Ref1 Average = 7.186412 V
Ref2 Average = 7.165433 V
The result shows a zero TC of the first reference at about 35 °C, a nice result. The real chip temperature will be a little higher, but not much (non-A version, long legs, 4 mA * 6.6 V = 26 mW).
The second reference was overcompensated. After some iterations of fine tuning it arrived at a compensation resistor of 16.91 Ohm in order to have the same zero TC temperature as the first reference. The agreement between both references is about 0.07 uV/K or 0.01 ppm/K and stable.
From the tuning process i determined the TC of Ref2 Ube as -2,07 mV/K. Ube is 488 mV at 20 °C.
Both reference TC measurements exhibit a very similar curvature. Without the tuning resistor the zero TC temperature would be higher, maybe closer to the recommended LTZ1000 oven temperature.
Soon more precision resistors will arrive in order to complete the other four cells of the array. I hope the observed 1 ppm "hysteresis" will then disappear.

Regards, Dieter

PS:
Measurements are from a Keithley 2700 with 7706 multiplexer plugin and of remarkable quality for this so-called 6.5 digit meter. Each data point is the average of 30 samples of 5 PLC ("slow"). The calculated difference between the two LTZ1000 references has a standard deviation of 0.91 uV over the whole sweep.

dietert1:
About the curvature: There is a 1 ppm peak to peak range of about 8 K. That means in order to reach 0.01 ppm peak to peak the common oven needs to be stable to +/- 0.4 K, a fairly easy requirement.
Total power consumption will be about 5.5 mA per cell: 4 mA zener + 1.5 mA for opamp. Times 6 cells and at a supply voltage of 10 V this gives a total of 330 mW. In comparison the ADR1399 evaluation kit consumes about 600 mW.

Regards, Dieter

Kleinstein:
If the LTZ1000 is used with the intended configuration with the heater active, there is no need to really trim the TC of the unheated reference. A crude compensation (e.g. some 10 or 15 Ohm - less than actually needed to get zero TC) should be enough to reduce the sensitivity to the oven set point quite a bit (e.g. a factor of 2).

The problem with the extra resistor is that it adds another critical resistor and also makes R1 more sensitive. With a resistor of 12 Ohm one would have 10% of R1 and thus some 50 mV contribution to the reference directly proportional to the extra resistor. So this would be an attenuation factor of about 140. With a larger resistance the attenuation goes down. So one trades a reduction in the importance of the set point divider for an additional resistor, that is about as critical.

The good points about the oven set point is that one only needs a ratio and one can measure that voltage to check if the set point divider is actually drifting by a significant amount. So I would rather prefer the oven set point over the extra resistor to trim the TC.

For the hysteresis it may be worth limiting the heater power. In the standard circuit the heat up would quite fast and with high power. Except for a possible high power peak I don't see why an external heater is much different from the internal heater.  It can still be interesting if just an external oven help against the hysteresis, I would not expect it to do.

dietert1:
Yes, for a single LTZ1000 the discussion will be different.
In my case using an external oven saves on circuitry and power consumption. And the oven is needed anyway for the filter caps and the 7 to 10 V gain stage.
Meanwhile i moved the complete analog temperature controller of the ADR1399 setup inside the oven. This way it works much better than an external controller, for example an Arroyo TecSource.

Regards, Dieter

dietert1:
An oven is under construction as well. The oven is a Weidmüller Klippon K6 and measures 18 x 14 x 10 cm. Until now it contains nothing but two film capacitors 380 uF 400 VDC. In order to see their leakage current they have to be at constant temperature. Maybe for a Sallen-Key filter or for a PWM filter.

Regards, Dieter