I would like to know whether the offset voltage of an AD8228 chip is a specific value or it depends on some factors?
And I'm also interested in how can I measure it because it is so small.
Thanks in advance~
Thank you for your interest in ADI products. The offset voltage of AD8228, just like other inamps and opamps, is an internal property of the part. To make it simple, the offset voltage is due to some mismatches of the transistors inside the amplifier. Many fabrication techniques have been developed to minimize offset voltage but it is still inherent in any amplifier. Over temperature, the offset voltage varies and this variation is described by the Offset Drift parameter in voltage per unit of temperature (usually in uV/degc or nV/degC).
The total referred-to-output offset (or total output error) of an instrumentation amplifier may be subject to other variables such as power supply variation and common-mode voltage. To measure the Offset Voltage as indicated in the datasheet, we must employ a two-point measurement technique so that other error contributing factors are cancelled. First is to tie both inputs of the inamp to ground, then set the gain to 10 and measure the output. Then adjust gain to 100 and measure the output again. Use the following formula to get the actual Offset Voltage, referred to input:
Vos(RTI) = (Vo2 - Vo1) / (G2 - G1)
Vos(RTI) = (Vo2 - Vo1) / 90
I hope this helps.
So if we only talk about the input offset voltage, it is only the function of temperature, right?
Generally speaking, that is right. The cause of the input offset voltage is the mismatches of the internal transistors and the offset drifts over temperature.
Thanks again for your support.
Since the features of the offset voltage is clear, I would like to know is there any measure can be taken to eliminate the offset voltage and its drift of AD8228?
The offset voltage can be measured by shorting the inputs of the part together and tying them to ground. If the amplifier was ideal, the output of the AD8228 would be exactly 0V. In practice, the offset voltage will appear at the output of the device. If you measure the offset voltage of the device, then add a signal to the amplifier input, you can subtract out the known offset voltage from your measurement to get a more accurate value for your input signal. How you accomplish this can vary, depending on your application, but you can use relays, a mux channel etc to accomplish the switching between input signal and nulling condition.
Some DMMs have enough resolution and accuracy to measure VOS directly at the output of the part, especially if you are in a gain of 100. What equipment do you have available?
I have a DMM-- Agilent 34401A.
Your suggestion seems unfamiliar for me. Could you explain it more detailed?
Can you try shorting the input to ground and measuring the output voltage with your DMM? What voltage do you get?
I haven't tried yet. Now I just want to learn how to measure it (Neil and you have told me above) and how to compensate it for AD8228.
If you have a known offset voltage, there are a couple of ways you can compensate for this and obtain a more accurate measurement. The most common way is to compensate for a known offset voltage is to simply subtract out the known offset voltage in the digital domain. You could also apply a nulling voltage to the reference pin of the AD8228. If you go this route make sure you drive the reference pin with a buffer, or other low impedance source, to avoid degradation in CMRR performance.
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