LT 1028 OP AMP QUESTION

[daverave]

i want to use the LT 1028 in my surf pi but i cannot get the output to zero using the offset preset...i can only get it to about 0.6v...anyone throw any light on this please ???

[Davor]

In LT1028 sheet it says:
"The input offset voltage of the LT1028/LT1128 and its drift
with temperature, are permanently trimmed at wafer test-
ing to a low level. However, if further adjustment of V OS is
necessary, the use of a 1k nulling potentiometer will not
degrade drift with temperature."

Linear Technologies gives the inner structure of LT1028 in http://cds.linear.com/docs/en/datasheet/1028fc.pdf

[daverave]

not sure what to do davor...i tried a 1k preset as the offset control but still stuck on 0.6v..it seems some op amps are not that easy to use.

[kt315]

you tried without coil and MOS-FET, only opamp itself, without a signal on his input?

[daverave]

Hi kt315

i tried with coil connected as the damping resistor is connected across coil plug...should i just try and setup LT 1028 with just damping resistor connected without coil ???

[kt315]

yup, only desolder MOS-FET too, leaving the damping resistor. you will check opamp without signal, in full static state. then try
to tune offset voltage.

[daverave]

ok thanks for that kt315

[daverave]

ive now zero volts on the output of the LT 1028....i just had to change a resistor value...i cannot now see any difference between the LT 1028 and the NE 5534...think i will stick with the NE 5534 op amp in the future !! im enjoying experimenting with the surf pi and trying to make small improvements to try and get that extra depth !! where i live the gold rings are very deep now and not even the dual fields are deep enough so im trying hard to get that bit extra depth which hopefully will give me that little bit advantage

[6666]

.i just had to change a resistor value

Which one, what value ?

[daverave]

R11 22k changed to 150 ohm.

[6666]

R11 22k changed to 150 ohm.

Thanks.

[Davor]

The link above contains a schematic with no resistor, e.g. 0 ohm. And a 1k trimmer.

[daverave]

thanks for that davor....i feel the LT 1028 in this application is not much more advantage over the NE 5534....and the LT 1028 is a lot more expensive...if anyone can tell me different then please tell me your views on this ???

[kt315]

how about to choice the opamp not per the noise but per Slew Rate?
your 'very good small noised' LT1028 has 13V/us slew rate, while for example AD845 has 100V/us. LT1028 is best for VLF
design, but for pulse induction you have still to think what is 'good'...

[daverave]

Hi kt315
i think your right !! does anyone use the AD845 in pulse detector ??? does the fast slew rate allow for early sampling ??