ADS1235: Is this a wrong circuit

Sunny Qin

Part Number: ADS1235
Other Parts Discussed in Thread: ADS1262

Hi Team,

From ADS1235 datasheet, I found a AC bridge excitation circuit as below:

From the circuit we can found that the REFP and REFN signal will also change with excitation direction change, that means Vref=REFP-REFN will change from 5V to -5V, but Vref should be >0.9V, so I am wonder if this is a wrong circuit?

Thanks very much.

Regards, Sunny

over 3 years ago

0 Bryan Lizon86 over 3 years ago

TI__Mastermind 35141 points

Hi Sunny,

As the datasheet says in Section 8.4.3., "The ADC compensates the reversed polarity conversion by internally reversing the reference voltage." So the ADC is always seeing a positive reference voltage even when the excitation voltage is reversed. Note that this functionality is specific to the ADS1235, and is not necessarily possible with other ADCs.

-Bryan

0 Sunny Qin over 3 years ago in reply to Bryan Lizon86

TI__Expert 4890 points

Hi Bryan,

Many thanks for your comments, one more question may still need your help:

Since most of bridge only output 10mV signal @5v excitation, even with gain of 128, ADC input is only 1.28V, while reference is 5V, so it will lose some effective resolution, So can we use resistor divider to offer reference for ADS1235 as below:

For example, if I choose R1=R2=R3=1k, then reference input is 1.66V, do you think this will improve effective resolution for 3 times?

Thanks again.

Regards, Sunny

0 Bryan Lizon86 over 3 years ago in reply to Sunny Qin

TI__Mastermind 35141 points

Hi Sunny,

Do you need to increase the effective resolution of this system? Adding a resistor divider eliminates many of the benefits of using a ratiometric reference, which means you are actually adding error to your system by implementing the resistor divider.

You can review the following Precision Labs modules to learn more about this topic and why it is not always necessary to maximize the ADC's full-scale range:

-Bryan

0 Sunny Qin over 3 years ago in reply to Bryan Lizon86

TI__Expert 4890 points

Hi Bryan,

That`s quite clear, thanks a lot.

Regards, Sunny

0 Sunny Qin over 3 years ago in reply to Bryan Lizon86

TI__Expert 4890 points

Hi Bryan,

One more question may need your help: if we do AC bridge, can we use MCU`s IO to control the AC drive? as I don`t know the principle of internal reference reversing, so I don`t know if intern reference will automatically reversing then we use MCU`s IO? or we must need to use ADS1235`s IO to drive the AC bridge?

Thanks.

Regards, Sunny

0 Bryan Lizon86 over 3 years ago in reply to Sunny Qin

TI__Mastermind 35141 points

Hi Sunny,

Reversing the internal reference only occurs when AC excitation mode is enabled in the device. In other words, you cannot reverse the reference voltage manually with the ADC. The only ADC that I am aware of that has this capability is the ADS1262.

You could use two additional external switches to also switch the reference voltage if you wanted to use an MCU's IO to control the AC excitation. Just make sure that during the switching process the ADC's operating voltage requirements are all met.

-Bryan

0 Sunny Qin over 3 years ago in reply to Bryan Lizon86

TI__Expert 4890 points

Hi Bryan,

Many thanks for your comments, so do you mean auto reference reversing will only happen in AC excitation mode(that means we must use ACX1/2/3/4 to drive the bridge)?

and you mean if we must use MCU`S IO to drive bridge, then we need one more switch to keep the reference always positive right?

Thanks.

Regards, Sunny

0 Bryan Lizon86 over 3 years ago in reply to Sunny Qin

TI__Mastermind 35141 points

Hi Sunny,

Correct, the VREF reversing is only active when the AC excitation is turned on in the ADC. So if the customer wants to control the AC excitation with an MCU, then they will also have to use another switch to make sure the voltage reference has the correct polarity at all times.

-Bryan

0 Sunny Qin over 3 years ago in reply to Bryan Lizon86

TI__Expert 4890 points

Hi Bryan,

Got it, it`s very clear, many thanks for your help.

Regards, Sunny

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ADS1235: Is this a wrong circuit