Analog to squared signal, need help...

Status
Not open for further replies.
Nigel Goodwin said:
3V is fine as a Vref (and you can easily adjust most precision references to be 3V), but you were talking about mV levels before - do have any idea what the levels are going to be?.
Click to expand...

I'm sorry if i'd missed out any info before this. The mV i was talking about is the sensitivity as before this i'd made an experiment and obtained the result that the sensitivity of the load cell (different model) is around 0.1mV/kg.

Nigel Goodwin said:
Sorry, what has sleep mode got to do with anything? - that's only to reduce power consumption when it's not being used, it won't improve conversion in any way.
Click to expand...

From what i read from the Microchip datasheet and some other manual (Section 17.22), "SLEEP and IDLE modes are useful for minimizing conversion noise because the igital activity of the CPU, buses and other peripherals is minimized."

And in Section 17.25 Design Tips
3. Put the device into SLEEP mode before the start of the A/D conversion. The RC clock source selection is required for conversions in SLEEP mode. This technique increases accuracy because digital noise from the CPU and other peripherals is minimized.


Zane

I'm learning to fish, but it takes patient...
 
I just got the specs for my load cell. It stated the output at rated Load is 3mV/V ± 0.25%. If my load cell could weigh up to 5tonnes, which means the output voltage signal is 3mV???

If i'm to set the Vref+ for my PIC to 3V, seems like i'm gonna amplified it 1000 times?

Nigel,

For PIC16F877A, it is a 10bits A/D. And the working register is a 8bits register. I did read from your site regarding to this but i don't really understand it. Would a 10bits converted result fit in 8bits? If i'm to transmit via RS232, means there's more problem i'll be facing soon?
 
Audioguru,

From the specs i read as posted above, is it possible to amplify signal from mV or maybe less than 1mV for 1000times higher? Since that the max output 5tonnes equivalent to 3mV/V.. Thanks
 
Zane83 said:
Audioguru,

From the specs i read as posted above, is it possible to amplify signal from mV or maybe less than 1mV for 1000times higher? Since that the max output 5tonnes equivalent to 3mV/V.. Thanks
Click to expand...
Of course you can amplify the signal. But I don't undestand its sensitivity spec. it will have an output of a lot more than just 3mV with a tonne or more, won't it?
The spec should not say 3mV/V, it should say 3mV/kgm or something like that.
 
audioguru said:
Of course you can amplify the signal. But I don't undestand its sensitivity spec. it will have an output of a lot more than just 3mV with a tonne or more, won't it?
The spec should not say 3mV/V, it should say 3mV/kgm or something like that.
Click to expand...

From what i understand from the spec, it seems to me like its 3mV (max output) if the weight is 5tonnes (max load applied). Hence, if the load is lighter, the output voltage will be lesser too. Anyway, i'd attached the spec. here (its the left most specs - 5000kg). Maybe you wanna have a look? If i'm mistaken, do correct me. Thanks.

I'd tried to build an amplifier which can amplify signal up to 1000 higher. This works only for signal higher than about 1mV. As i remember, you told me that op-amp has its some like threshold voltage? Do you know any op-amp which can amplify such low voltage? Actually i just bought a TL084.

Thanks
*i wonder why its not displayed in thumbnail form first*
View attachment Load Cell Specs.zip
 
Last edited:
I wish you attached the file as a compact GIF or PNG file type, instead of zipping up a huge JPG that is supposed to be for photographs.

The load cell needs an exciting voltage, 10V is recommended. That is the "V" in its output spec. So with 10V of excitement then its output at rated load is 3mV x 10V=30mV. Use an amplifier with a gain of only 100.

A TL08x opamp has a gain of 200,000 up to 20Hz. Its gain is 100 up to about 32kHz. You could use two of the opamps in series, each with a gain of 10 and the total gain will be 100 up to about 320kHz.

Opamps don't have a "threshold".
 
audioguru said:
I wish you attached the file as a compact GIF or PNG file type, instead of zipping up a huge JPG that is supposed to be for photographs.
Click to expand...
Sorry for that.. my comp seems 'sick' lately that its abit out of my control.

audioguru said:
The load cell needs an exciting voltage, 10V is recommended. That is the "V" in its output spec. So with 10V of excitement then its output at rated load is 3mV x 10V=30mV. Use an amplifier with a gain of only 100.
Click to expand...
This the V of 3mV/V means the excitation voltage...? Now i got it..."[/QUOTE]

audioguru said:
A TL08x opamp has a gain of 200,000 up to 20Hz. Its gain is 100 up to about 32kHz. You could use two of the opamps in series, each with a gain of 10 and the total gain will be 100 up to about 320kHz.

Opamps don't have a "threshold".
Click to expand...

Just now i tried to simulate a circuit, it seems like it cant be amplified as the input signal is too small. Maybe i should say operating signal? Just like a diode which will only operate after 0.7V Si.

Luckily i bought a TL084 with 4 opamp in it where i still got 2 to spare. But may i know why i cannot amplify it straightaway with gain of 100 instead with 10 10? Sorry as i dont understand...
 
Opamps gradually reduce their gain for higher frequencies to avoid oscillating. The TL08x opamp can amplify with a gain of 100 up to only 32kHz. It amplifies with a gain of 10 up to 320kHz.
You know the max signal risetime that is required, so convert it to the opamp's flat frequency response required, but I forgot the calculation.

Attach a sketch of the opamp circuit you simmed to see why it didn't work. It probably didn't have a negative supply.
 

I'll try to do with it. Hopefully it works. I'd attached the circuit i done using the TL084. I wonder why it didnt work. I checked the pin already and the connection seems alright (for me ).

Got class now! Catch ya later~! Thanks!

 
Sure it is amplified. The inverting opamp circuit has a gain of 100 as determined by the ratio of the resistors. But its input signal is way too big at 1V, so its output is trying to go to 100V which it can't.

The inverting circuit has an input resistance of only 1k ohms which loads-down the output of the sensor. So make a non-inverting amplifier that will have a very high input resistance.
 

I'm sorry audioguru.. i tried increasing value of the resistance you'd told me. I even tried using others TL084 model and changing values. Somehow i could not get the result yet. From the image i attached, the output is zero instead! I wonder where i had done wrong now. Feeling hopeless with even a simple amplifying circuit.. Sigh...

 
Instead, if i use the amplifier TL082CP only i could be able to get desired result from the simulation.. maybe software problem in the component?
 
Your simulation software is stupid. The opamp should have a big output if the DC output of the generator is 0V and is 1V AC at 60kHz like is shown.
 
audioguru said:
Your simulation software is stupid. The opamp should have a big output if the DC output of the generator is 0V and is 1V AC at 60kHz like is shown.
Click to expand...

Haha.. i think something wrong with my software too.. Anyway thanks for being patient with me.
 
Status
Not open for further replies.

Similar threads

R
  • Solved
Analog voltmeter does not go back to zero
Replies
12
Views
1K
Nigel Goodwin
S
  • Question
Need help with spst toggle to momentary
Replies
5
Views
3K
crutschow
O
  • Question
Reading Digital + Analog Data from Sensors / Bus
2
Replies
26
Views
4K
Tony Stewart
Z
Need some help for a FM LIFI communication
Replies
1
Views
2K
rjenkinsgb
R
  • Question
(Another) analogue voltmeter does not go back to zero
Replies
7
Views
891
JimB
Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…