ADS 1248 -with Dual Power supply-(+/-2.5V)- Internal Reference connection

Amit Patil57

Prodigy 150 points

Other Parts Discussed in Thread: ADS1248

Hi,

I'm using ADS 1248 -with Dual Power supply-(+/-2.5V).

I like to know about Its VREFCOM terminal connection.

As per datasheet

1) We have to connect VREFCOM pin to Mid supply of AVDD & VSS as per pin description.(As per page no.: 8)

2) We have to connect VREFCOM pin to AVSS (which is -2.5V )through 10E Register.(As per page no.: 29)

Please share view? which one has to follow?

Regards,

Amit

over 9 years ago

0 Joseph Wu over 9 years ago

TI__Guru* 97595 points

Amit,

As you mentioned referencing the datasheet, the VREFCOM pin requires a low impedance connection to an AC GND node such as AGND or AVSS.

The reason for the low impedance connection to an AC GND node is for reference stability. For that reason, I generally think it's better to connect the output to GND, because I don't always know the output impedance coming from the source driving AVSS.

I would note that the ADS1248EVM shows that the VREFCOM is always connected to the AGND node. With a unipolar supply (+5V), VREFCOM is connected to AVSS. With the bipolar supply (±2.5V), this is still the midpoint analog ground. Just note the VREFCOM is connected to AGND through a short, low inductance connection.

Joseph Wu

0 Amit Patil57 over 9 years ago in reply to Joseph Wu

Prodigy 150 points

Joseph,
Thanks

I'm using this for universal Analog input ..
For three wire RTD i'm using Ratiometric apporch as given in application note below;
e2e.ti.com/.../495449

As I'm using dual power supply for ADS1248, Where i should connect Rbias resiter Whether it should be connected to AVSS or GND?
Right know in my PCB i have connected it to GND .
But i'm not getting Ref voltage across Rbias extacly?
Rbias=820 ohm
Excitation current is 1mA & i'm testing it with PT100.
I'm Expecting Rbias drop=(1ma+1mA)*820 ohm= 1.64V.
But i'm getting it 1.3V only, what could be the reason?

Amit

0 Joseph Wu over 9 years ago in reply to Amit Patil57

TI__Guru* 97595 points

Amit,

I'm not positive which application note that you are referencing. It's likely that you are reading SBAA180, using the 3-wire RTD circuit in Figure 2. I'll put a link to it below for any future readers of the post:

www.ti.com/.../sbaa180.pdf

If you have a bipolar supply, you tie the negative side of RBIAS to AVSS, or the lowest potential. You lose the compliance of the current as the voltage on the pin supplying the IDAC current rises closer to the positive supply. In the datasheet, there are two plots in the Typical Characteristics curves showing the IDAC current compliance as the voltage on the pin rises.

If you want to use the circuit from the application note, you will need to connect the negative input of RBIAS to AVSS. This will give the IDAC current source plenty of headroom for operation.

Joseph Wu

0 Amit Patil57 over 9 years ago in reply to Joseph Wu

Prodigy 150 points

Joseph,
Thanks...
I did the experiment as suggested by you. Negative side of Rbias is connected to AVSS. I'm getting results for PT100.
For PT100,
Rbias=820 ohm
Excitation current is 1mA
Expected Rbias drop=(1ma+1mA)*820 ohm= 1.64V. I'm getting it 1.661.
24 bit count observed are linear.

For PT1000,
Rbias=820 ohm
Excitation current is 0.1mA
Expected Rbias drop=(0.1ma+0.1mA)*820 ohm= 0.164V. But I'm getting it 1.502V..
I have increased this Resistor for PT1000 case from 820 ohm to 4.7 Kohm. to have Reference input voltage > 0.5V
Then i'm getting 1.807V which is also more then what i expected as below.
Rbias drop=(0.1ma+0.1mA)*4700ohm= 0.940V

What could be the reason to have this voltage observed across Rbias is irratic or more than what i expect?

Amit

0 Joseph Wu over 9 years ago in reply to Amit Patil57

TI__Guru* 97595 points

Amit,

My guess is that you have something wrong in the setup. First, can you please draw up a basic schematic of what you have? I think I already know, but it always helps to have a schematic to be clear.

Second, can you read back the register values to make ensure the value of the registers? Report them back with the next post. It's important to read back all the registers to make sure there aren't any unexpected registers set to make other parts of the device active.

I've run similar experiments, and haven't had any problems setting up the correct currents values and excitation current outputs.

Joseph Wu

0 Amit Patil57 over 9 years ago in reply to Joseph Wu

Prodigy 150 points

Joseph,

Thanks..

Please find attached block schematic for PT100 and PT1000 range. We have shown schematic only related to these two ranges.

For PT100, we have following observation

--For PT100 Rbias resistor value is 820 Ohm.

--Excitation current is 1mA.

--External Reference voltage with this Rbias resistor is 1.88V. ( measured between Pin REFP0 & REFN0).

--24Bit ADC count observed is linear for -50C to +250C

But For PT1000, we have following observation

--For PT100 Rbias resistor value is replaced with 4K7 Ohm.

--Excitation current is 0.1mA.

--External Reference voltage with this Rbias resistor is 1.807V. ( measured between Pin REFP0 & REFN0).

--24Bit ADC count observed is not random for -50C to +250C

Settings PT100 & PT1000 are same except Excitation current as maintained above.

As you suggested I will get back with Registers settings

Our queries are:

1. For PT100, Why External reference not as per expectation exactly i. e. Expected Rbias drop=(1mA+1mA)*820 ohm= 1.64V.

But still we are getting readings with this.

2. For PT1000, Why External reference not as per expectation i. e. Expected Rbias drop=(0.1mA+0.1mA)*4700 ohm= 0.94V.

Why we are not getting 24Bit ADC count for PT1000.

0 Amit Patil57 over 9 years ago in reply to Amit Patil57

Prodigy 150 points

0 Joseph Wu over 9 years ago in reply to Amit Patil57

TI__Guru* 97595 points

Amit,

As you mentioned, I think that the register setting values will be helpful. When you are able, can you please read back the registers? This will make sure that the values are as you think. With the correct settings, there shouldn't be any reason for the excitation currents to be an unexpected value.

Regardless, there are a couple of things to check. I'll list them below.

- Why do you have a resistor at VREFCOM to your -2.5V supply? I would make sure this is a simple short. The capacitance should have less than a 10 Ω resistance to the AC ground, with the series resistance, it may affect the stability as well.

- There is a voltage divider at R1/R2. Is this input connected? This will definitely affect the current going to RBIAS.

- There is a resistor labeled PTC at the 3rd connection to the RTD. What is this value? If it is high, then this might affect the IDAC compliance voltage. However, this will reduce the current, not increase it.

Here are two other comments that might affect accuracy. These issues won't cause extremely large errors, but may introduce smaller errors.

- Are you driving IDAC0 and IDAC1 to the AIN0 and AIN1? It would be better to use a separate pin to drive the RTD connection. If you don't, you add the error in the mismatch between the two 249 Ω resistors.

- With this three wire measurement you want to take a measurement, swap IDAC0 and IDAC1, make a separate measurement, and then average the two. There is a max 0.15% mismatch error between the two IDACs and you may eliminate this error through this chopping technique.

As I mentioned earlier, you'll want to check the register values and report them back here. Again, there isn't any reason that the IDAC currents should put out a value different than expected.

Joseph Wu

0 Amit Patil57 over 9 years ago in reply to Joseph Wu

Prodigy 150 points

Joseph,

Thanks
Followings are reply
-- Input not connected at Voltage divider R1 /R2 .
-- PTC is of 11 Ohm.

A) For PT100 followings are registers settings:

MUX0 = 0x41, MUX1 = 0x20, SYS0 = 0x31, Vbias = 0x00, IDAC0 = 0x06, IDAC1 = 0x01

With this setting, we are getting 1mA Excitation current. But with this current & 820 Ohm Rbias resistor, we are getting External Reference voltage as 1.88V. As per calculation this reference voltage should be 1.64V [ Rbias * (Iexc0 + Iexc1) = 820 Ohm ( (1mA+ 1mA) = 1.64V]

We are getting linear & approx. stable ADC 24 bit count with above register setting & reference voltage. The P100 mv input between AIN0 / AIN1 pin is stable & also Reference voltage between REFP0 / REFN0 pins is stable.

B) For PT1000 followings are registers settings:

MUX0 = 0x41, MUX1 = 0x20, SYS0 = 0x31, Vbias = 0X00, IDAC0 = 0x02, IDAC1 = 0x01

With this setting, we are getting 0.1mA Excitation current. But with this current & 7500 Ohm Rbias resistor, we are getting External Reference voltage as 1.82V. As per calculation this reference voltage should be 1.5V [ Rbias * (Iexc0 + Iexc1) = 7500 Ohm ( (0.1mA+ 0.1mA) = 1.5V]

We are getting random & very unstable ADC 24 bit counts with above register setting & reference voltage. The P1000 mv input between AIN0 / AIN1 pin is stable & also Reference voltage between REFP0 / REFN0 pins is stable.

Our queries are:

-Why PT1000 not working even though we are getting 0.1mA excitation current, PT1000 stable mv input at ADC AIN0 / AIN1 channel & stable reference voltage between REFP0 / REFN0 pins.

We have additional following observations:
1. When we bypasses i.e. removes CD4051 mux, then we are getting stable Reference voltage as per calculations

For PT100

i.e. [ Rbias * (Iexc0 + Iexc1) = 820 Ohm ( (1mA+ 1mA) = 1.64V]

ADC 24bit count remains same as previous i.e. count linear & approx. stable

For PT1000

i.e. [ Rbias * (Iexc0 + Iexc1) = 7500 Ohm ( (0.1mA+ 0.1mA) = 1.5V]

ADC 24 bit count remains as previous i.e. count is non linear & unstable.

2. When we connects -5V supply at VEE pin of CD4051 mux, then we are getting stable Reference voltage as per calculations i.e. 1.64 for PT100 & 1.5V for PT1000. But in this case we are not getting ADC 24bit counts for PT100 as well as for PT1000, even though there is stable mV input between AIN0 / AIN1 pins & stable reference voltage between REFP0 / REFN0 pins..

If required we can send more details on email. Will you please share your email ID?

Amit.

0 Joseph Wu over 9 years ago in reply to Amit Patil57

TI__Guru* 97595 points

Amit,

I don't see anything wrong in your basic setup. The device should be set up correctly. If you are able to get the PT100 example to work, then you should be able to get the PT1000 example to work. Just replacing RBIAS from an 820Ω resistor to an 8200Ω resistor should be exactly the same. I would note that earlier you said this was a 4700Ω resistor for the PT1000 case.

My first suspicion of a problem would be the multiplexers. I would try bypassing them to make sure there isn't a problem, with some sort of leakage current. I know that these muxes should have a low leakage, but perhaps one is damaged and the leakage is higher than expected.

I would also remove any extraneous resistors on the board. Remove R1 and R2 and disconnect pin 5 from the CD4052. This will prevent any leakage at the REFP0 node. With a precision voltmeter, you could determine the current running through PTC. I would also remove the 124Ω resistor also. I don't see the purpose of this resistor.

I would also bypass the CD4051 for the same reason. If there is some sort of leakage then this might be a source of error. If you want you can remove the series filtering resistors as well, but you may need them to verify the currents through the RTD and RBIAS.

Going back to your questions an comments, I do have a few responses.

You've mentioned that your PT1000 measurement is random and unstable, what values are you getting? Can you take a set of 100 points are report the result? It helps to know how far off the measurements are and what the noise level really is.

You mention that if you remove the the CD4051 you get a stable reference voltage (or at least the correct voltage) Doesn't that seem to imply that this might be the issue? If you connect the -5V for VEE, then you get the correct reference values? If you have noise, then this may take more to determine if there is something to be done with it.

Again what values are you getting as a result? It's important to get the raw data (output codes) to see what noise you have. If you have a larger set of data, you can see if this noise comes from line noise or some other source.

I've sent a request to connect. You should be able to send messages through E2E that I'll receive.

Joseph Wu

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ADS 1248 -with Dual Power supply-(+/-2.5V)- Internal Reference connection