Subject: Clarification Needed on PDbar Pin Behavior at ±2.5V Supply
Hi everyone,
I am working with [OPA858-Q1] and have a question regarding the PDbar (Power Down) pin.
The datasheet mentions that for normal operation, the PDbar pin must be connected to the positive supply. However, under the “Power Down” section, the behavior at ±2.5V supply seems to differ.
- What should the PDbar pin be connected to when operating at ±2.5V to enable the op-amp?
- Could someone explain the reasoning behind this behavior?
- will there be any change in voltage at input inverting terminal if we are giving PDbar to positive supply?
I appreciate any insights or references to clarify this.
Thank you!
Hello deepthia,
From their datasheet the PDbar should be below -0.7V for the “Power Down” state. Any voltage applied to PDbar above -1.85V would be in normal operation.
Pin 5 would be the reference for the negative voltage, so Pin 5 + 1.8V would be your enable point and pin 5 + 0.65V would be normal operation.
The gap you are seeing is the hysteresis in the on/off on the pin. See section 6.5’s table in the same datasheet for the raw values where they have pin 5 as the reference
yeah so we can connect short PDbar to positive supply for normal operation right?
or do we have to connect PDbar to 1.3 exactly for normal operation , when i am operating with ±2.5 supply
Your thinking is correct, if you run the positive reference (pin 7) to the shutdown pin (pin 8) you will have normal operation.
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But , if we change PD bar pin voltage do the offset voltage or input inverting terminal voltage changes.
Do we connect different voltage to PDbar pin different from supply , or else is it always shorted to positive supply .
The voltage on PDbar should not affect input offset voltage or inverting terminal voltage.
If you do not intend to use the PDbar function, then always connect PDbar to the positive supply.
If you do intend to use the PDbar function, then to enable the device, PDbar should be at least 1.8V above the negative supply rail, and to disable, it should be no more than 0.65V above the negative supply rail. In each case, the best would be to set PDbar to V+ or V-, as nesessary.
On a separate note, I read on page 20 of the datasheet that the thermal pad should always be connected to the negative rail, regardless of whether you are using a single supply or a split ±2.5V supply. If PDbar is connected to ground while using a split supply, this could significantly affect the part’s behavior. Be sure to check that it is connected properly to the negative supply rail on your pcb.
From page 20 of datasheet:
Also, please carefully read and follow the layout guidelines found on pages 26-27 of the datasheet. Layout is critical to optimal performance (or even functional performance, for that matter) of this part. Any deviations from those recommendations can significantly affect performance.
