I would want to use the AD7793 with two 3-wire RTDs (100 ohms).
Is it possible?
the compensation resistor in your schematic will not null the lead resistance. The current will flow from the IOUT pin through the RTD. So, the voltage measured by the ADC will be the RTD resistance + lead resistance. There is no current flowing through the compensation resistor (ignoring leakage current) so the resistance has no affect.
When the second excitation current is used, this second current flows through the lead resistance and the voltage generated will null the lead resistance in series with the RTD.
To null the lead resistance, a 4-channel device such as the AD7794 could be used. One channel is connected across the RTD. A second channel could be used to measure the voltage generated by the lead resistance. Rlead in the attached pdf is the lead resistance and channel AIN3 is used to measure the voltage generated by the lead resistance.
The AD7794 has the same feature set as the AD7793. The only difference is that the part has extra channels.
do you plan on using the 2 excitation currents for each RTD so that the lead resistance is nulled as per the AD7793 datasheet? If so, external muxes are needed to direct the currents between RTD1 and RTD2. See the attached diagram.
If you dedicate an excitation current to each RTD, the connections are much simpler and the external muxes are not needed. However, in this case, you will need to null the error due to the lead resistance via another method.
I think the solution with an excitation current to each RTD is the better than to use muxes.
See the attached diagram.
How can I null the error due to the leads resistance?
Is it enough a Rcomp as in the attached diagram AD7793_1_1_with_Comp.RTDs.pdf ?
Hi. I'm student. I am researching a subject "RTD Application Using the AD7793". Did you do it? Can you send to me: diagram, initial value to register in AD7793 (Mode register, Configuration register, IO register,...) ? I have tried but i didn't finish.
I don't speak english well so i apologize to you for my discourteous.
My mail: firstname.lastname@example.org
there is a diagram attached to this discussion which shows the connections between the RTD and the AD7793. The output data rate required depends on your application. However, an output data rate of 16.6 Hz gives sim 50/60 Hz rejection which may be required for your application. What excitation current do you plan on using. For example, consider a 4-wire RTD and an excitation current of 420 uA. For a pt100, the max resistance (at 600 deg C) is 313.6ohms. So, the max voltage generated across the RTD is 420 uA x 313.6 = 0.1317V. If the gain of the AD7793 is set to 16, the max voltage seen by the modulator is 16 x 0.1317 = 2.1074V. The reference resistor should be selected so that Rref x 420 uA = 2.1074V. So, Rref should equal 5.11K.
A current of 210 uA is generated by setting both excitation currents to 210 uA and then directing both currents to IOUT1 or IOUT2.
I did but i didn't read correct value in data register.
Example: when I set IOU1 = 210uA, the current in IOUT1 flows through R=10kOhm (I test IOUT1), I read Ur = 2.6V not Ur = 10000*0.00021 = 2.1V??????
(IOUT1=210uA -----> R=10kOhm) -----> GND --- i measure voltage by Digital Multimeters )
Thank you very much for attending!
Setup 7793: AIN1, External Ref, gain= 16, 16Hz, IOUT1 = IOUT2 = 210uA --> Iref = 420uA --> Vref = 0.00042*5140 = 2.158V (but i measure, Vref = 2.9V?????(by digital mutilmeter))
Read data: 23.7 deg C, not (111/100 -1)*0.00391 = ~29deg C (pt100 ~ 111Ohm).
(Rt = data_read / 16777215 /16)*2.158 / 0.000210 )
where are my fault?
the excitation current is typically 210 uA. Over temperature, it will increase. However, it should increase to about 240 uA. It should not be higher than this. So, with a reference resistor of 5.14K, the reference voltage should be 5140 x 2 x 210 uA = 2.158V. Even with temperature, the voltage should not increase to 2.97V. Can you check that the reference resistor is indeed 5.14K.
Can you explain how you derived the equation for degC?
Rt = Ro * (1 + A* t + B*t2 + C*(t-100)* t3) = ~ Ro*(1+ A*t)
--> t = ((Rt/100) - 1 )/A ;// A = 0.0039083~0.00391
Is it correct?
2. When i read data in data register, sometimes error is + - 1 degrees? why?
When read data:
- check status register:
--> if value of status register == 0x08 (AIN1)
--> then write communications register (0x58) --> read data in data register
3. What is offset register and full-scale register acting? I don't know how to use them?
Thank you very much!
can you check your formula with an RTD manufacturer. A pt100 has a resistance of 100 ohms at 0degC and 138.51 ohms at 100 degC. Using the formula that you have attached, the result is correct at 0degC but contains an error at 100degC. For this reason, can you check it.
The AD7793 has offset and gain registers. These registers can be used to remove the internal offset and FS errors. You can perform internal calibrations to remove these internal errors. System calibrations can also be performed. So, the offset and FS Error of the system can be calibrated by the AD7793.
Hi I got a 36 channel RTD Temperature system and my previous reference design is using AD7793. Previously the reference design is only using 4 channels but now the new system requires 36 channels and its a 4 wired RTD solution.
Can I check if there is any better part to use instead of AD7793 to minimise the number of ADCs on my board.
The AD7794 has 6 differential inputs. However, one of these inputs is also the exc current. So, one AD7794 can support 5 4-wire RTDs. An external mux is required to direct the exc current to each of the RTDs. The lowside of all the RTDs can be connected to the REFIN+ pin. An external precision resistor is needed to generate the reference voltage. So, the exc current will flow through the RTD and through the precision resistor i.e. a ratiometric configuration is needed.
To accommodate 36 channels, 7 AD7794 devices are needed. The exc current from one one part needs to be used to supply 2 AD7794s to limit the ADC count to 7.
Thanks and Best Regards,
Thank you for the advise.
Is there an app note on this or a reference design that you may have come across?
You can probably refer to this:
This is similar to what you are looking for except it uses an AD7193 and an external current source.
Hope this would help.
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