OPA376: measurement circuit for quiescent current

Tachibana-san,

You may NOT connect op amp in an open-loop configuration (no feedback) as shown on your circuit schematic because shorting the inputs will result in the internal input offset voltage being gained up by the open- loop gain thus forcing the output to one of the rails where datasheet condition of Vout=Vs/2 will NOT be satisfied.

To measure IQ, place the op amp in buffer (follower) configuration, by shorting output to inverting (negative) input, while driving the non-inverting (positive) input to mid-supply.  The easiest way to accomplish this configuration is by using the dual +/-2.75V supplies and grounding the input.  You may then determine IQ by either placing an amp-meter or small (100ohm) resistor,  in series with positive or nagative supply pins and use amp-meter or volt-meter, respectively, to take the measurement. Since there should be no output current, Iout, under quiescent condition, there is no need to include RL in IQ measurement (it makes no difference).

Hello Lis-san,

Thanks for your advice.

According to your explanation, measurement circuit for quiescent current is as follows, isn't it?

Are the specified values in the datasheet also measured with those circuit?

And when I would like to measure the quiescent current under the following condition,
should I apply Vs/2+1.3V (=3.75V) at V+ terminal in the left circuit?
"TA = 25°C, IO = 0, VS = 5.5 V, VCM < (V+ ) – 1.3 V"
Please let me confirm to be sure.

Best regards,
M. Tachibana

Tachibana-san,

Your IQ measurement circuit configurations as shown above are correct and we measure IQ in production in a very similar way.  

Having said that, as long as your input common-mode and the output voltage are within the OPA376 linear range (see CMRR and AOL condition column), the quiescent current will be the same.

Hello Lis-san,

Thank you for your prompt reply.
I understood about the measurement circuit and IQ within linear range.

By the way, how much is the quiescent current in non-linear range such as near the both rail?
Does it decrease or increase relative to IQ within linear range?

Best regards,
M. Tachibana

Tachibana-san,
If the op amp operates in a non-linear range, the current you measure is NOT quiescent current anymore and it may vary differently in comparison to real IQ depending on the process technology and circuit topology used. Since we mostly characterize only linear op amp performance, we do not have such data to share with you but knowing that OPA376 is a CMOS part, I would expect the current to stay the same or go down.

Hello Lis-san,

Thank you very much for your help.

Since our customer pointed out that the current consumption tended to decrease less than IQ when measuring it near the rails, so I asked you questions.
When the voltage of output signal is near the rail, I guess MOS transistors of internal circuits such as current mirror will operate in the non-saturation region. As a result, I think the current consumption would decrease.

Is it correct?

Best regards,
M. Tachibana