I am using this device in a 2nd order unity gain Sallen-Key low pass filter. R1=R2 = 100k, C1 = .22uF, C2 = .47uF. Vcc = 2.5 volts.
I am seeing about a .250 volt drop across R2 at about 2volts which means I have 2.5 uA disappearing at the non-inverting input. I don't suspect a leaky capacitor (but haven't ruled it out). This is about 10 times the spec I believe.
I have washed the board and the filter works. The excessive current however is a problem.
Any hints/help is appreciated.
Hello Eltury,
I am assuming the following circuit, with
R1=R2 = 100k, C1 = .22uF, C2 = .47uF. Vcc = 2.5 volts. Gain = +1
Agreed that 2.5uA is too much for the input current of the amplifier - if it is within the legal input voltage (CMVR) range.
What is the DC level at the input? If Vin DC is 2V, then you are violating the upper common mode range of the amplifier.
The maximum input to the amplifier is 900mV below V+, or 1.6V on a 2.5V supply.
You are probably seeing the effects of violating the common mode range. Move your average DC value to below 1.6V or use a low noise R-R input device like the LME49721, LMP7731, OPA322 or OPA365.
Also, if you are using polarized capacitors, make sure the "+" end of the cap points to the most positive voltage. Tantalum and electrolytic capacitors are diode-like and can leak quite heavily (uA's) when reversed. Use non-polarized film or ceramic for lowest distortion.
Also keep in mind loading by the DMM or scope probe when measuring. A 10M DMM/Scope probe will drop about 40mV across the 200k, or about 300mV with a 1M "1:1" scope probe. You can see this effect by watching the output DC level while probing the circuit.
Regards,
