[SOLVED] analog to digital converter adc0809

[saurabheda]

1. can adc0809 take negative values of input voltages ?
2. and if it takes, then i doubt that a zero volt at input would not give 0x00 at the output ?
what would be the output in this case ?

 

[shamikrudra]

Features
■ Easy interface to all microprocessors
■ Operates ratiometrically or with 5 VDC or analog span
adjusted voltage reference
■ No zero or full-scale adjust required
■ 8-channel multiplexer with address logic
■ 0V to VCC input range
■ Outputs meet TTL voltage level specifications
■ ADC0808 equivalent to MM74C949
■ ADC0809 equivalent to MM74C949-1
Key Specifications
■ Resolution 8 Bits
■ Total Unadjusted Error ±½ LSB and ±1 LSB
■ Single Supply 5 VDC
■ Low Power 15 mW
■ Conversion Time 100 μs
read full datasheet

 

[saurabheda]

i have gone through the datasheet..
so cant we give a sine wave input to adc0809..?

 

[shamikrudra]

Yes it takes sine wave as input.

 

[ark5230]

The sine wave input has to be level shifted so that the input to the ADC always remains positive. For example if I have a sine wave with peaks between -2V to +2V then shifting it by about +2.5 volt will make the signal between +0.5 o +4.5V. This signal can safely be used with the ADC 0809.

 

[saurabheda]

yeah..thats fine..and then can i use a clamper circuit(using diode) for this purpose..?

 

[ark5230]

If the signal is clearly below +4.5 V DC, it is OK, as a precaution use of such a diode is advisable.

 

[poorchava]

AC bipolar signals can be input to single supply ADC without a problem. Just add a voltage divider on ADc input to set the bias of 1/2 VCC (or 1/2 Vref if it's different than VCC) and then couple AC signal to the input using capacitor. In that case 0v of input signal the output of ADC is only determined by bias setting resistors. That's and 8 bit ADC, so in case of biasing it with 1/2 Vref you get 1/2 of full scale which is 0x7F. The maximum negative swing would be 0x00 and maximum positive swing would give 0xff

Sadly, this methos doesn't work with DC signals, because of capacitive coupling.

 

[ark5230]

Did any body try this method with ADC 0809? It seems little involved due to reasons related to other components in the circuit under use.

 

[muralicrl]

Hi,

ADC0809 can take the analog input from 0 volts to 5 volts only. Definitely it will not accept negative voltages. DC shifted any waveforms (sine, cos..) will be accepted.

1. can adc0809 take negative values of input voltages ?
2. and if it takes, then i doubt that a zero volt at input would not give 0x00 at the output ?
what would be the output in this case ?

 

[poorchava]

Did any body try this method with ADC 0809? It seems little involved due to reasons related to other components in the circuit under use.

I'm pretty sure it will work. I think the only reason for this solution not to work would be if the adc had low input resistance which i'm almost sure it doesn't.

I can't tell how will your circuit behave without seeing schematic.

 

[anoopsharma007]

hiii everyone
i am generating a 3V sine wave and giving to adc0804 and the output of 0804 is connected to dac0808 .the output i m getting on cro is a +ve signal only. -ve part is clipped. what should be the error.??? is it 0804 negative input????????/ Please help

 

[ark5230]

hiii everyone
i am generating a 3V sine wave and giving to adc0804 and the output of 0804 is connected to dac0808 .the output i m getting on cro is a +ve signal only. -ve part is clipped. what should be the error.??? is it 0804 negative input????????/ Please help

If the AC signal is not level shifted and suitably attenuated, it is likely that the 3V AC swings between +4.24 and -4.24.
As a result, if the ADC is 5V full scale, the positive part will be converted to digital and that is what you see on the CRT after DAC. The negative part is not understood and handled by ADC so it is being shown as zero or some flat line. This could be reason if all the rest is fine and taken care appropriately.

 

[anoopsharma007]

guys i solved the concept of a bipolar signal...
according to my knowledge adc 0804 can not convert a digital value of a -ve signal.
so if we are applying a +2V , -2V peak to peak sine ,triangular or square wave, 0804 will only convert data for +ve side.
when -ve signal is coming 0804 is giving 0 value.
what i do , i just store the +ve signal and during -ve wave i plotted the -ve side by inverting the +ve stored signal.
but this program is working only for 20HZ to 300hz signal....

 

[ark5230]

Level shifting is a better approach.
regarding limitation of frequency to 20Hz to 300 Hz signal, if you use higher frequency crystal that the micro-controller supports this range will be increase. Additionally if the program is consuming more time in other operation that code can also be tried to optimize number of instructions executed. If you are using delay loops, excess delays can be reduced. This will increase the range of frequencies.

 

[anoopsharma007]

thankx i completed the project with help of clamper circuit....it is working perfect on almost every frequencies...