MONOCOIL SUCCESS

[moodz]

Sure will, money doesn't grow on trees. I've got a couple of STP9NK50ZFP mosfets that I will try, looking at the specs it has a lower capacitance than the IRF740.



Mick

Mick .. you probably already know but there are protective diodes on the inputs of the THAT if the input voltage to the amp exceeds the amplifier power supply the diodes will conduct ... this could also upset the blocker .... you should maybe consider using a series resistor of upto 10k on each input to the amp to limit any current flows if something bad happens. This will marginally degrade the noise performance but will save the amp.

moodz

[mickstv]

Mick .. you probably already know but there are protective diodes on the inputs of the THAT if the input voltage to the amp exceeds the amplifier power supply the diodes will conduct ... this could also upset the blocker .... you should maybe consider using a series resistor of upto 10k on each input to the amp to limit any current flows if something bad happens. This will marginally degrade the noise performance but will save the amp.

moodz

I guess it will be time to look for a replacement, any ideas ?



Mick

[Midas]

No worries moodz, I understand it's busy time of year.

Thanks mick, look forward to seeing your results.

I'm going to be using the lme49990 mainly because I read they were about the best available for a traditional design in a thread somewhere on here, so I'd already purchased a bunch. Seems quite a bit better than the THAT to me. Significantly better CMRR (137db), slightly lower noise, good bandwidth. Pretty old thread so I've got no idea if its the ducks nuts any more, technology marches on and all, so probably not.

[moodz]

No worries moodz, I understand it's busy time of year.

Thanks mick, look forward to seeing your results.

I'm going to be using the lme49990 mainly because I read they were about the best available for a traditional design in a thread somewhere on here, so I'd already purchased a bunch. Seems quite a bit better than the THAT to me. Significantly better CMRR (137db), slightly lower noise, good bandwidth. Pretty old thread so I've got no idea if its the ducks nuts any more, technology marches on and all, so probably not.

Cheers Midas .... I will get some results but Mick will probably beat me to it. The LME49990 certainly has some pretty good looking specs but it is an OP-AMP not a diff AMP like the THAT .... snippet from the 49990 spec sheet below if you want a diff amp ... I am not saying dont use it though ...( if you got em ... smoke em )

moodz

[moodz]

I guess it will be time to look for a replacement, any ideas ?



Mick

I reckon just make up a diff amp using ne5534 ... ???

moodz

[mickstv]

I reckon just make up a diff amp using ne5534 ... ???

moodz

Got plenty of them. I also have a INA133UA sitting in the parts bin, unity gain diff amp. Might be ideal if it doesn't have input diodes.

http://www.ti.com/lit/gpn/ina133



Mick

[moodz]

The input diodes are not a show stopper .. for instance if you run the front end off 8 volts the voltages on the output of the blocker will be from 1 to 17 volts approximately ( due to leakage of the high voltage flyback ) ... so you would want to run the THAT off say 0 and +20 volts to give you a bit of headroom. The 8 volt supply is the analogue ground reference point. The THAT can be powered off upto +-20 volts ( ie 40 volts ) so if you did have a 40 volt supply to power the THAT you could run the frontend off something as high as 18 or 19 volts ... As a rule of thumb the zener diode on the blocker should be twice the supply voltage but not higher than the Gate breakdown voltage of the blocker mosfet.

with an 8 volt supply and the THAT running from +20 volts the protective diodes were never forward biased.

PS you must make sure the damping is correct so the coil flyback does not cycle negative either ....

moodz

[Midas]

The LME49990 certainly has some pretty good looking specs but it is an OP-AMP not a diff AMP like the THAT .... snippet from the 49990 spec sheet below if you want a diff amp ... I am not saying dont use it though ...( if you got em ... smoke em )

Yep I realize that. That was pretty much the circuit I was planning on using. I thought we'd already given up on single chip diff amps. But if you do know of\find one with specs as good as the lme49990 I'm all ears. I'm sure I can find another use for the lme's. Whatever I use though I'm actually hoping on keeping the magic smoke inside the little black box.

[mickstv]

The input diodes are not a show stopper .. for instance if you run the front end off 8 volts the voltages on the output of the blocker will be from 1 to 17 volts approximately ( due to leakage of the high voltage flyback ) ... so you would want to run the THAT off say 0 and +20 volts to give you a bit of headroom. The 8 volt supply is the analogue ground reference point. The THAT can be powered off upto +-20 volts ( ie 40 volts ) so if you did have a 40 volt supply to power the THAT you could run the frontend off something as high as 18 or 19 volts ... As a rule of thumb the zener diode on the blocker should be twice the supply voltage but not higher than the Gate breakdown voltage of the blocker mosfet.

with an 8 volt supply and the THAT running from +20 volts the protective diodes were never forward biased.

PS you must make sure the damping is correct so the coil flyback does not cycle negative either ....

moodz

Thanks for the info.



Mick

[6666]

I reckon just make up a diff amp using ne5534 ... ???

so you reckon that would be the way to go Moodz ?

[moodz]

so you reckon that would be the way to go Moodz ?

Well what I meant was you could use them if you have them on hand .... they are fast enough and reasonbly low noise / low cost ... depends if you demod then amplify or amplify then demod.

moodz

[Aziz]

Hi all,

I personally wouldn't use the discrete instrumentation amplifier version. Any imbalance of the source impedance (source resistance, source capacitive load) and/or the used resistors in the discrete version (requires better than 1% tolerance), will radically reduce the common mode rejection ratio (CMRR). Particularly much more, if you have high frequency signals (no doubt, we have it here). The CMRR is frequency dependent!!!

The money is well invested in an instrumentation amplifier IC (laser trimmed internal resistors).

Aziz

[moodz]

Considering that the unbalanced input of the amplifier in most PIs have very little, if any common mode rejection ( thats why you need all the elaborate shielding on the coils ) the discrete differential amplifier will be fine. You actually only need about 16 db of CMRR whereas an "unmatched" discrete differential amplifier will achieve usually better than 30 db.

The bottom line is that for this application you dont need some fancy pants CMRR rejection level .. you will never be satisfied with the performance if you indulge in catalogue engineering ( there is always another chip with better specs ).

Good engineers make what they want out of what they can get.

Make it work then make it better .. not the other way around.

moodz.

[Midas]

Considering that the unbalanced input of the amplifier in most PIs have very little, if any common mode rejection ( thats why you need all the elaborate shielding on the coils ) the discrete differential amplifier will be fine. You actually only need about 16 db of CMRR whereas an "unmatched" discrete differential amplifier will achieve usually better than 30 db.

The bottom line is that for this application you dont need some fancy pants CMRR rejection level .. you will never be satisfied with the performance if you indulge in catalogue engineering ( there is always another chip with better specs ).

Good engineers make what they want out of what they can get.

Make it work then make it better .. not the other way around.

moodz.

Hi Moodz,

I'm not sure what you mean when you say you don't 'need' fancy CMRR. Are you saying that past 16db you stop seeing any benefit in noise reduction?

I was thinking the more CMRR the better which is one of the reasons I decided to go with the discrete opamp solution. My plan was to add in a multi-turn trimpot to one or two of the resistors where I was hoping I'd be able to to adjust it its to a better CMRR than the THAT1510. Perhaps its not worth it.

So what did you mean when you said:
"The THAT amplifier is not the best here ... use opamp DC diff amp."
?

Midas

[Aziz]

Hi guys,

no doubt, you can use the discrete version too. You just need a compensation network for best CMRR (crucial point in differential amplifier applications). This means adjusting the source impedance (source resistance and capacitance) and the amplifiers resistor network.

The complete instrumentation-amp IC's reduces noise, pcb space and cost of course.

Aziz