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Issure about testing OPA615

Luxing Wang
Prodigy 160 points
Part Number: OPA615


Hi everyone, I met some problem when I build the circuit to test OPA615.

I only want to use the OTA function not SOTA part. Therefore, I just leave the pins(4,7,10,11) related to SOTA part disconnected, and build the others just as figure 39 in datasheet. In addition I connected Pin 2(Emitter)  directly to the ground. For pin 12 (Collector), I used a resistor of 50 as the load. 

However, when I powered this chip without injecting input signal, I noticed that there was some DC value at the input pin (Base) and the output pin (Collector). For different chips, the value will be different.

In addition, for one chip, it turned out quite confused condition, which is there are quite large current flowing out of the power supply, around 150mA. However, after I injected a input signal like 30mV with a 25 resistor in series at the input pin(Base), that chip turned out to work properly. 

My question is: 

First, how to deal with the unused pins for SOTA part, so that there is no impact on my OTA function part. 

Second, where is the DC value from under input pin open circuit? In addition, why there is a big current flowing in the chip? 

I wonder if the chip cannot be used as the input pin is under open circuit.

I am looking forward to your reply. Thanks so much.

  • 0
    TI__Genius 12631 points

    Hello,

    It is not good to leave unused input pins floating. To resolve this, can you please tie pins 10 and 11 to GND. You can also tie pin 7 (Hold) to GND. The output (pin 4) can be left floating.

    Once this is done you can leave the emitter tied to GND and collector tied to GND through a 50 ohm resistor.

    What is the value of Rq you are using?

    Please use this TINA-SPICE circuit to also simulate your circuit so you can compare your measurements with the simulations.

    -Samir

    OPA615_OTA.TSC

  • 0 in reply to Samir Cherian
    Prodigy 160 points

    Thanks so much about your reply. Actually, I have simulate its spice model. However, in the model, when I connect all the unused pins(pin 4,7,10,11) to the GND, and just leave emitter tied to GND and collector tied to GND through a 50 ohm resistor, I can observe there is still something strange. First, I set Rq equal to 300 ohm. If I just leave Base pin disconnected, the voltage on this pin is -1.57V. There is a big current of -35mA flowing into the chip from collector. In addition, there is a big current of -63mA flowing into the chip from emitter.  Only if I inject some signal like 30mV into Base pin through a 25 ohm resistor, the current will drop to the normal. At this time, emitter current becomes 2.8mA and the collector current becomes the normal as input voltage times transconductance.

    That is why I wonder if the base cannot be disconnected. However, how to solve it?
    I am looking forward to your reply. Thanks so much
    Lucy
  • 0 in reply to Luxing Wang
    TI__Genius 12631 points
    Lucy,
    There has to be some impedance connected to GND otherwise the base current has nowhere to go. You can do this by attaching a resistor to GND...just make sure that the source you are connecting to the base can drive the load.
    -Samir