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Nov 29 2008, 7:47 AM
Hi all i am working on a project and i would like your help here is theme

To perform a measurement, we use a method called “oscillometric”. The air will be pumped into the cuff to be around 20 mmHg above average systolic pressure (about 120 mmHg for an average). After that the air will be slowly released from the cuff causing the pressure in the cuff to decrease. As the cuff is slowly deflated, we will be measuring the tiny oscillation in the air pressure of the arm cuff. The systolic pressure will be the pressure at which the pulsation starts to occur. We will use the MCU to detect the point at which this oscillation happens and then record the pressure in the cuff. Then the pressure in the cuff will decrease further. The diastolic pressure will be taken at the point in which the oscillation starts to disappear.

So far i have only developed softeware in c just interfacing ADC 0831.
so please help he in making amplifier for transducer.Ic AD620 and OPA2277 are not available in Pakistan can any 1 help me replacing following ckt with LM324

Nov 29 2008, 8:41 AM

Dec 2 2008, 7:27 AM
I want to know that is it possiable to implement following using MC51 in c.

continusly check for voltage from ADC2 if it is greater than 4v then save the value from ADC1??

Dec 2 2008, 10:44 AM
yes thats possible.

in C you get what you write. thats the reason C became so popular in embedded.
Dec 3 2008, 10:52 AM
What logic is required I mean algo
Dec 3 2008, 7:19 PM

so please help he in making amplifier for transducer.Ic AD620 and OPA2277 are not available in Pakistan can any 1 help me replacing following ckt with LM324

ur ckt diagram suggests that the AD620 is a instrumentation amplifier so to replace it by a lm324, u will have to rig up an instrumentation amplifier using three op-amps from a single package of lm324. As far as the OPA2277 is concerned, u can try replacing them directly with the individual op-amps of the lm324. I used the term 'try' because each op-amps have their on unique characteristics and the OP series are generally high performance OP-amps. So simply replacing them with a general purpose op-amp like 324 will have some trade-offs. U will have to check the output practically to know the difference.

What logic is required I mean algo

ur diagram suggests u are outputting an A.C. voltage to the ADC0. Can u clarify what type of waveform ( sine/pulse/square) appears there?.any particular reason to output a second order differential of the input?

Dec 4 2008, 7:15 AM
its needed like this.we have to measure ossillations .going further when cuff has enough pressure that no blood flows through arm there would b no oscillations but when pressure is slowlly decreased a time comes when oscilation began to appear and grow when they reach their maximum pek lets say max amplitude (4v we set ). we measure dc pressure from ADC1.
type of wave expected are shown in PICTURE
Dec 4 2008, 9:05 PM
well you mean you have to measure voltage of signal too at the same time. you might need a faster ADC for faster conversions. what is maximum frequency of the i/p signal?
Dec 5 2008, 9:44 AM
Sorry i didnt understand i/p signal.Well i am using MC51 and serial adc ADC0831 and pressure sensor SCX15dn.And here is the web from where i am translating it from mega32 to mc51.


i didnt mean to measure voltages at same time program remains in function which measures AC signals if they rise above 4v then go to function that records pressure.this is to b achived by two ADC.so we dont have to measure Pressure aswell as AC signals.
Dec 5 2008, 4:21 PM
from the application ad the details related on the link provided, i presume the frequency of the oscillations is equal to the pulse rate which is around 70 ~ 80 bpm for a heathy soul.
So, u can just output it to ur ADC and sample the signal for every small interval of time and check the amplitude . As the frequency is very low ( ~1~2 Hz) , u can safely read the samples using a conventional 8051.

P.S. If u read the article in the link, u will get a fair explaination on deciding the sampling rate.
Dec 9 2008, 2:09 PM
Yes but i need to design an amplifier that is capable of measuring small oscilations and feed it to ADC but problem is that in PAKISTAN there is no MPX2050,OPA2277 or AD620 aso i have to design whole thing my self i came to ckt that is in my sensor's datasheet(SCX15DN)that gives pressure reading to ADC0831 nw i have to amplify the signal and again ,couple it and feed to another adc.As it hav DC level of 2.5v we chk when it exceeds 4V.Nw how dose freq is related to samples ??(Sory if this is a silly Q)
Dec 9 2008, 7:23 PM
first try to understand the signal conditioning circuit:
the first op-amp is the amplifier which amplifies the output from the pressure transducer to the required voltage level and is fed to one adc channel.
the other two stages using opa2277 are actually band pass filters tend to eliminate the dc component from the signal along with any additional noise from the sensor. In the last stage, a fixed known D.C. component of +2.5V is added so that the oscillations occur in a non-zero environment.
Now, what u have to do is to first practically rig up the analog circuit along with the pressure sensor and practically check if u are getting the required A.C.and D.C. outputs. I would suggest u start with the interfacing once that is confirmed.

As for ur second query , yes, the sampling frequency should be always very high compared to the A.C. wave frequency.
Dec 15 2008, 7:36 AM
thank u sir.
mr arun told to
use fig B.But i am using ADC0831 so is it better to use schematic in fig D or Fig B for Bp calculations and sensor i have got is SCX15DN and all circuits are for SCX05Dn and SCX01Dn.Here it makes a confusion that variation of the sensor is from 0 to 60 mv (SCX15Dn) which accordingly assigns 12mv to each 1 Psi.and 0 to 18 mV in case if SCX01DN.
these ckts are fine if i use SCX05Dn.Well Question is that ckt given is for
SCX05Dn but i am using SCX15DN which allows me to have 6mV/Psi and it is only available.For medical use 300mmHg is ab 5.8 Psi.nw for fig B given CKT comes to Voltage 3.5 when sensor voltage reaches 60mV(max pressure).in my case when 300mmHg is achived ckt should be at 3.5V when voltage of sensor reaches to 34.8mV.

By the Formulla

what modifications are required to change the ckt in Fig D or in
Fig B if i am using SCX 15Dn.and how can i calculate overall gain
of ckts in fig b and D
Dec 16 2008, 4:47 AM
Afnan plz quit one of the two threads you are running for the same topic.
Abandon this thread and continue the discussion in the other one.
Dec 16 2008, 9:32 AM
Sure ill soon quit other fourm nd ill continue this 1.
Dec 22 2008, 11:17 AM
can any 1 tell me dat is it necessary to use op2277 in above fig.Can i use Lt1014 i use op2277
what i want to do is dat measure small ossiliations
Jan 6 2009, 7:34 AM
Ok please reply.
What makes precsion op amps different 4rm each other.Lt 1014 and opa 2277.

and if i use band pass filter would ir mater if i use band pass filter
Jan 6 2009, 2:29 PM
well filters are for a purpose, usually designed keeping the next stage in mind.

regarding replacement of LT1014, I would recommend you to download datasheet of both Opamps and review the electrical and operational characteristics. or just see the features of LT1014 and find an identical one from national semiconductor's website.
Jan 6 2009, 7:14 PM
The LT1014 is a bipolar input opamp whereas the opa is a jfet.
See if you can find LT1055 or LT1022.
As a last option you may try using LF353.
Jan 12 2009, 8:37 AM
ok ill track them.as there are filters and gain would it b same for them too or i got to disign whole filters again.

what bipolar and jfet op amp differ in ?
Jan 12 2009, 12:55 PM
You may use the same circuit.
Basically input impedance and noise levels would be different for jfet and bipolar.
Jan 13 2009, 5:27 AM
and what about frequency resopnce if its 1-4 hz would it matter for bipolar or jfet
Jan 13 2009, 2:13 PM
At such low frequencies the input noise density would be a bit greater for jfet-input opamps.
Don't worry too much about these now.Keep looking for the best precision opamp you can lay your hands on.
And meanwhile experiment with various locally available alternatives.Use a socket so that you can easily swap different opamps.
Use precision 1% or better resistors in the opamp ckt to keep overall noise low.
Jan 29 2009, 7:17 AM
i came uopn various op amps.ive noticed that some have single supply and some hav doual ie+_5 V.Why?
Jan 29 2009, 4:53 PM
if the input signal to be amplified or compared ( that is mostly why op-amps are generally used) tends to fall into the negative region ( below 0V), then u invariably require a dual supply op-amp for the purpose to preserve the input information through to the output.
single supply op-amps can be used for the other cases where the input is strictly restrained in the positive voltage region.The single supply op-amps also tend to consume less current and are used in battery operated appliances where power consumption is a major issue.
Feb 10 2009, 5:56 PM
is it like that u got to use if u measure input impedance within some range.i used lt1014 on proteus and got result as in picture.i have a question that adc 0831 can measure when woltage rises on above any spicified voltage.what is input impedence

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