LF398 DC Zeroing

Sinoj B S

I'm using LF398 in my application which is powered from +/-12V. We have a +10V reference voltage on board. The datasheet says to connect the DC zeroing potentiometer to connect to V+ line. I have two questions:

Q1: If there's a fluctuation on +/-12V line, will it affect DC output offset? Will it be a good idea to connect the DC offset zeroing potentiometer to the +10V reference voltage instead of connecting it to V+ line?

Below is a copy of section DC and AC Zeroing from the Datasheet.

[DC zeroing is accomplished by connecting the offset adjust pin to the wiper of a 1 kW potentiometer which has one end tied to V+ and the other end tied through a resistor to ground. The resistor should be selected to give »0.6 mA through the 1k potentiometer]

Q2: What is the nominal hold capacitor value and how it is calculated (I think it's a compromise b/w hold-time and leakage)? What dielectric material is recommended for this (available in SMD)? Any part number for 220nF or 470nF?

Thanks | Sinoj

over 12 years ago

0 Paul Grohe over 12 years ago

TI__Guru 55126 points

Hi Sinoj,

Actually, fluctuations in the supply should not cause too much effect on the output - it should actually get better because you are tightening up the internal matching. The resistors and the input stage are both sourced from the same supply - so they will track. But this is ONLY if you wish to cancel out the native LF398 input offset.

The internal input stage has two 2K tail resistors to V+. What the external resistors are actually doing is balancing out the input stage currents by paralleling one side of the input stage tail resistors.

Pin 2 has 9K series resistor connected to the tail resistor of one of the input pairs. By applying a voltage to this point, you can add/subtract current out of this node and "balance" the input stage currents to zero the offset voltage. Even though the 12V "fluctuates", both sides of the input stage should both have the same errors - so the net result is zero. This is why you want to completely balance the input stage.

But beware that the external resistors will not have the same tempco as the on-die resistors, so this "balance' will now be temperature dependent. If you are trying to use the offset adjust to compensate for external offsets not caused by the LF398, you end up "bending" the input stage too much to force the offset and it no longer accurately tracks.

As for the 10V reference...The 12V is still powering the "other "side of the input stage - so you will not have true "balance". The 10V reference would not track fluctuations in the 12V supply, which could actually force an error across the input stage.

As for the capacitor - you need to look at the first few graphs and make your selection among the tradeoffs. Large caps hold longer but take longer to charge. Small caps sample faster, but "droop" more. Expensive poly caps work best, but are large and expensive.

Poly film caps are available in SMT up to around 1uF - but they are not going to be in the convenient tiny 1206 or 0805 sizes. They are going to be the size of large tantalums.

See the Cornell Dubilier (CDE) "FCP or FCA" series, and Panasonic has some, too - but be prepared for sticker shock....Just Google "Surface Mount Film Capacitor" to find more sources.

Also see Bob Pease's Capacitor Soakage article:

http://portal.national.com/rap/Application/0,1570,28,00.html

Figure 7 shows all the various capacitor dielectric types. It's fairly self-explanatory.

Regards,

Amplifiers

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LF398 DC Zeroing