Understanding the Autozero Function

Note: This tutorial applies to the 34401A, 34970A, and 34980A products.

The autozero function provides a more accurate measurement by removing the internal voltage offset due to temperature. First, the input terminals are measured as well as the internal offset voltage. The offset is then subtracted from the input to deliver a more precise reading.

If speed is of utmost importance, or your application does not call for high-resolution accuracy, you can almost double the measurement speed simply by turning autozero off.

Autozero Settings

Note: Autozero is used for dc voltage, dc current, 2-wire ohms, and temperature measurements only. Autozero is automatically enabled for 4-wire ohms and RTD temperature measurements.

Autozero can be enabled/disabled in various ways:

The command [SENSe:]VOLTage[:DC]:ZERO:AUTO [, (@ )] provides three modes: {OFF | ONCE | ON}. ON (default) causes a zero reading to be taken after each input measurement. Selecting OFF or ONCE will disable autozero. OFF does not issue a new zero measurement. ONCE issues an immediate zero measurement. When disabled, a new zero reading is taken each time the function, range, or integration time is changed.

If you are taking measurements over a short interval, the drift is not likely to change significantly. Autozero does not need to be enabled. However, if you are reading over a long period of time (>10 minutes) it is a good practice to conduct periodic ONCE measurements. Autozero ONCE is also useful when the configurations were set some time ago, and now you want to take a new burst of measurements and remove any drift that occurred over time.

Cycling power or sending a *RST command will enable autozero.

The MEAS? and CONF commands automatically enable autozero. However, you can indirectly enable/disable autozero with these commands when you set the resolution. If less precision is required, then autozero does not need to be enabled. You can achieve up to the respective resolution for each range and have autozero disabled automatically.

Maximum resolution that disables autozero:

MEAS? and CONF affect autozero settings differently. MEAS? will default any previously coded autozero settings to ON. The only way to use MEAS? and disable autozero is to specify resolutions with equal or less precision than that defined in the above table. A better method for disabling autozero is to use the CONF statement first and then set autozero to OFF. Now you can use INIT + FETCH? or READ? to conduct your measurement.

To minimize drift in your measurements, you should warm up your machine for at least one hour. Autozero enabled will remove the drift. Autozero disabled offers greater measuring speed, but cannot offer the same error protection. If you set autozero OFF after warming up the instrument at a calibration temperature of ±1°C, and conduct the measurement in less than 10 minutes, you should expect an additional error of up to 0.0002% range and +5uV.

The graph to the right was derived in a test program for the 34980A using autozero ON vs. OFF with multiple readings on one channel. DC VOLT, range 10, NPLC 1, and function calls INIT and FETCH? were used. This verifies that autozero OFF is approximately twice as fast as autozero ON.