Electronics > Projects, Designs, and Technical Stuff
Voltage dividers on power supply rails
new299:
Hi all,
I've been presented with a design that has voltage dividers on the power rails. These are used to take +/- 5v down to +/- 3v (100,150Ohm IIRC). They are used to drive the rails of an opamp. The opamp is a precision transimpedance amplifier for signals in the pico to nanoamp range (LMP7721). It draws about 1mA. My tests show that current can vary by about 500uA with input signal (mostly when it starts hitting the rails). Current draw is supposed to vary by ~1mA with temperature.
The LMP7721 datasheet suggests using a regulator and filter network on the rails, but not a voltage divider.
I've always understood that using voltage dividers on the supply is bad practice. It does appear to work with this part. But I've never seem it done. I can't see any advantage over using a voltage regulator (aside from a slight cost advantage).
I feel like using voltage dividers here is bad practice and has the potential to cause other issues down the line.
Am I just crazy? Can anyone show me an example where this has been done and was a reasonable design choice? Or have any other input as to why this might be a good/bad idea?
bob91343:
The problem with a voltage divider is that it has a high impedance, and thus its output voltage will vary with current load. This may not be acceptable for your design.
Depending on how much variation you find acceptable, there are several schemes that will work.
The obvious zener diode. Temperature instability and soft knee.
Series diodes, similar issues.
Active regulator. Not simple, takes many components.
In any case, if the load doesn't change much then a voltage divider may be the best solution.
mikerj:
What resistor values are used for the divider? If the current through the divider is significantly higher than the op-amp current then voltage change will be small, but obviously this increases power consumption.
Presumably the divider is well decoupled with a suitable capacitor? If not it's inviting performance problems such as instability.
Zero999:
It sounds like a bad idea. The LMP7721 is also only specified to a total power supply voltage of 5.5V and +/-3V is a total power supply voltage of 6V.
OM222O:
--- Quote from: Zero999 on August 03, 2020, 07:43:13 am ---It sounds like a bad idea. The LMP7721 is also only specified to a total power supply voltage of 5.5V and +/-3V is a total power supply voltage of 6V.
--- End quote ---
That's not correct. Page 4 of datasheet, absolute maximum ratings:
"SupplyVoltage (VS= V+ā Vā)(3) ā0.3 6.0V"
"(3)The voltage on any pin should not exceed 6V relative to any other pins."
so +-3V supply is within the specified limits, albeit a bit dangerous.
Navigation
[0] Message Index
[#] Next page
Go to full version