GoldExI : FAQ : AutoZero

Question

How does AutoZero feature work?

Answer

Sometimes the ADCs or preamplifiers have a shift of zero. It doesn't matter really and you don't need to care about this because GoldExI has special "AutoZero" feature that will help you to compensate this defect. When you check (click) the "AutoZero" check box in the Measurement window, GoldExI does the following:

Sets all connected currents (DACs) to zero.
Waits for DelayAfterSweep1 or DelayAfterSweep2 whatever is longer.
Measures all connected voltages, averaging them N times as precise as normally (normal averaging see in Averaging options). More exactly: it calculate the mean value among N voltages obtained using regular averaging.
Considers these measured values of voltages as a new zero for corresponding voltages.

All further measurements will automatically be accomplished relative to this new level. To reset this zero correction and provide voltages as they are, just check out "AutoZero" check box.

Comments

On step 1 GoldExI does not check whether the value of current really becomes zero or not. Thus, if you have DAC (not ADC) with substantial zero shift, take care! Also keep in mind that If you have external current sources connected to the sample, make sure that they are turned off or create no voltage across voltage channels.

The options for AutoZero can be set choosing Options | AutoZero from menu. The meaning of settings:

Show AutoZero warning -- if it is on, GoldExI shows warning which explains what exactly happens and what are the pitfalls every time you check AutoZero checkbox in the measurements window.
Ignore T-sensor voltage during AutoZero -- if it is on, AutoZero procedure ignores all voltages of the type T-sensor. This is useful when the temperature sensor is fed by the current which is not controlled by GoldExI. But the best solution is to have a switch which can short a sensor. Then you short this switch before checking AutoZero, and when AutoZero is complete, open it again. In this case you don't need to use Ignore T-sensor voltage during AutoZero and will have more exact temperature readings.
Redo on "DoIt" -- When you often change the gains (Ext. or ADC), it is necessary to do AutoZero again and again before each measurement. If you check Redo on "DoIt" check box, GoldExI will redo AutoZero every time when you push "DoIt" button. Note, however, that in this case T-sensor voltage will be always excluded from AutoZero-ing and AutoZero warning will not be shown. There are some pitfalls (see below).
Extra Averaging -- How many extra times AutoZero routine averages the values of voltages obtained using normal averaging procedure. It calculates the simple mean value.

Pitfalls

If you have a system/sample with non zero voltage even when all currents are set to zero, the "AutoZero" feature won't work properly.
- First example: the system with hysteresis. You can try to reset the system manually playing with currents in Measurement Window.
- Second Example: some external not controlled by GoldExI currents are injected into the circuit. You need to turn off all external sources before you check on "AutoZero".
- Third example: If some voltage channels are disconnected and you check on "AutoZero", the zero voltage of these channels will be a garbage. If you then connect these voltages to your sample and try to measure something, you will get an arbitrary voltage shift of these voltages. Thus, if you rearrange or connect new virtual voltages, reset and set "AutoZero" again.
Redo on "DoIt" may mess up your measurements, if you want to start from a particular bias point of the IVC. Suppose that by means of fine manipulations of bias current you put your bias point on some of the hysteretic branches of the IVC and want to trace this branch from current point up. So you checked "Set Start" and unchecked "Set Stop". Then you push "DoIt". If Redo on "DoIt" is on, AutoZero will reset all current to zero, removing the bias point from the proper branch and set it again to the previous value, which may correspond to another branch...oops!

See also: How does the Averaging work?

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