ADC Isolation part

Hi Manjunath,

Thank you for posting to E2E! If this is the only signal to be isolated, a one-channel device like the ISO7710 can be used here. This is a 2.25 to 5.5V part, so the 12V input signal will need to be reduced for compatibility with the isolator.

Regarding the 1.8V output, ISO7021 or ISO7041 support 1.8V, so even though the input signal would still need to be regulated, if the isolated side (Vcc2) is powered by 1.8V, the output signal level will be compatible with the 1.8V ADC input.

TI's digital isolators are designed with output buffers, so an external one is not necessary between the isolator and ADC. The system would be something like the block diagram below:


Please do let us know if the ISO7021 can be used (with the unused channel configured correctly). What is the datarate of this signal to be isolated?


Thank you for your time,
Manuel Chavez

Hi Manuel,

Thanks for the reply,

How to regulate the ADC level from 12 to 1.8?

Hi Manjunath,

No problem! My apologies for not clarifying sooner, but is the signal to be isolated a digital or analog signal? The previous recommendation I made was to isolate a digital signal with LOW = 0V and HIGH = 12V, like a RESET, CS, or data line.

If the 0V - 12V signal is an analog signal, the solution above will be insufficient to isolate the signal for input to an ADC. In this case, the analog signal requires isolation, but that is not a robust approach for signal integrity or cost.

A potential alternative is to isolate the data lines out of the ADC (SPI, LVDS, I2C, lines) instead of the analog input signal. Is this possible in your system or is this the original idea?


I apologize for the confusion.


Have a great weekend,
Manuel Chavez

And also we require 2 digital output pins, with a signal of 5V,

But my MCU out will be 1.8V.

What is the isolation you will suggest ua here,?

Hi Manuel,

I have updated the design, Please find it in the attached file.

GPIO's.pdf

For GPIO

Total 2 Digital input with 12V inputs and One Digital output with 5V output level.

1. Whether the design is sufficient?

2. 12V supply is Isolated but the 5V supply is Isolated as shown in the circuit, and shorted the Isolated and External GND(12V supply GND) together, will this work?

3. Or 12V also should be isolated?

Please suggest.

For ADC

You please suggest

[Same answer in the logic forum.]

The R34/R35 voltage divider does not generate a valid logic level. To set the B→A direction, drop R34; but you can connect DIR directly to GND.

Unidirectional downtranslation can be done with a buffer with overvoltage-tolerant inputs; you can replace U21 with a SN74LVC2G17-Q1.

CMOS inputs must not float; ground pins C and D of U22.

These three level shifters do not support isolated power supplies and require a common ground on both sides. And when you do connect all the grounds together, isolating the power supplies is pointless. If you do require isolation, then you must also isolate the three signals with a real isolator like the ISO7731-Q1. (It cannot go below 2.5 V or above 5 V, so you still need the level shifters.)

Hi Clemens Ladisch,

Please find the attached updated design.

8585.GPIO's.pdf

1. The R34/R35 voltage divider does not generate a valid logic level. To set the B→A direction, drop R34; but you can connect DIR directly to GND.

Manjunath : Noted.

 2. Unidirectional downtranslation can be done with a buffer with overvoltage-tolerant inputs; you can replace U21 with a SN74LVC2G17-Q1.

Manjunath : "SN74LVC2G17-Q1" doesn't do voltage translation from 5V to 1.8V.

3. CMOS inputs must not float; ground pins C and D of U22.

Manjunath : Thanks Noted.

4. We are currently looking for Digital Isolators for our apllications.

i. We need to isolate 2 Digital inputs whose digital lines voltage will be 12V

ii. and One Digital output whose voltage levels is 5V

iii. Main MCU accepts only 1.8V level.

So, for Digital inputs, Considering from Outside world, first I need to regulate the 12V levels to 5V levels and then I need to Isolate it and then again should do the voltage translation to 1.8V. I'm I right here?

For Digital Output, I need a Isolation and then Voltage translator to 1.8V.

Please check the attached schematic and review the same.

Manjunath

When you run the SN74LVC2G17-Q1 with V_CC = 1.8 V, it accepts 5 V signals at its inputs (see V_I in the Recommended Operating Conditions table), and outputs 1.8 V signals.

Your isolation architecture is basically correct. However, the INx pins are for signals going into the ISO chip, and the OUTx pins, for signals going out of it; you have to exchange them. (And the ISO7731-Q1 would be cheaper than the ISO7742-Q1.)

Thanks for the reply,

1. I have updated SN74LVC2G17-Q1 in the design(Assuming output levels will be equal to VCC and Input lines can handle a voltage upto 5.5V regardess of the VCC voltage).

I have also updated the directions of ISO7742-Q1

GPIO's_1.pdf

2. Regarding the ISO7731-Q1, This has 3 channels and in that 2 are forward direction and 1 in reverse direction. But We require 2 in Reverse direction(Digital Output lines) and 1 in forward direction(1 Digital output line).

Hence I have choosen ISO7742-Q1.

Please suggest

Manjunath 

The LVC1G17 can convert down, but not up. To shift the 1.8 V signal to 5 V, you still need the SN74LVC1T45-Q1.

The ISO7731-Q1 actually has one channel in the forward direction and two channels in reverse, exactly what you need:

Hi Manjunath,

Thank you for updating schematics and selecting the ISO7742. This is a newer isolator and works well in between CD40109B-Q1 and SN74LVCxxxx devices. The separation of grounds is also schematically correct, and I have a few minor notes:

1. R36 can be increased to 1kΩ to reduce transients due to ground voltage differences
2. The unused channel INB of ISO7742 (pin 4) should be connected to VCC_5V, its default output state. This pin and Enables 1 and 2 (pins 7 and 10) should be connected to VCC_5V with a 4.7kΩ resistor.
3. Isolation transformer 750315240 is good to use for this system, but it is meant for applications without an LDO, so the LDO may drop out if for any reason the SN6505 input Vcc voltage drops by >5%.

Hi Clemens,

Thank you for your responses in this thread! Your insight has been helpful and I appreciate your support over the weekend.


Respectfully,
Manuel Chavez

Thanks for the response.

1. In the design 12V supply is directly from external source and its not isolated, is that fine?

2. R36 can be increased to 1kΩ to reduce transients due to ground voltage differences

Manjunath : Noted and Updated

3. The unused channel INB of ISO7742 (pin 4) should be connected to VCC_5V, its default output state. This pin and Enables 1 and 2 (pins 7 and 10) should be connected to VCC_5V with a 4.7kΩ resistor.

Manjunath : Noted and Updated

4. Isolation transformer 750315240 is good to use for this system, but it is meant for applications without an LDO, so the LDO may drop out if for any reason the SN6505 input Vcc voltage drops by >5%.

Please find the updated schemtic.

2664.GPIO's_1.pdf

Manjuanath

Hi Manjunath,

Yes, it is okay for the CD40109B's 12V supply and input signal to be non-isolated.

This LDO selection is also better for drop-out margin, however best practice would be to choose a transformer with higher turns ratio like the 750316031. This transformer is acceptable to use for 5V:5V with LDO operation.


Thank you,
Manuel Chavez

Hi Manuel,

Thanks for the review comments,

It is Okay to go with "750315240" only, where it is 1:1.1, For 5V input, Output will be 5.5V, and the LDO drop out is 0.25V.

I have updated the design and kindly review the same for the final time(Isolated Power supply section is not included)

8562.GPIO's_1.pdf

One more question, My initial question was regarding the input ADC pin Isolation.
Two Analog input channels should be given,
each channels should be isolated.and with over voltage protection, since our main MCU accepts 1.8V voltage levles.

Can I same "ISO7742QDWQ1" Digital Isolator? and then How can I translate the 5V level to 1.8V level?

Please suggest on how to do it,

Thanks 

Manjunath.

Hi Manjunath,

You're welcome!

Manjunath G14 said:

It is Okay to go with "750315240" only, where it is 1:1.1, For 5V input, Output will be 5.5V, and the LDO drop out is 0.25V.

Manjunath G14 said:

One more question, My initial question was regarding the input ADC pin Isolation.
Two Analog input channels should be given,
each channels should be isolated.and with over voltage protection, since our main MCU accepts 1.8V voltage levles

Hi Manuel,

Thanks for the reply,

Regarding the transformer, I have understood,

Regarding the Digital I/O's I have added 4.7K to the INx pin,

Regarding the ADC, Please find the attached schematic and review the same. 

Digital IOs.pdf

Hi Manjunath,

I appreciate your confirmation. Regarding the ADC schematic:

• Are the 5V analog signals (ADC inputs) differential signals or are they two independent channels into the ADC?
• Are the signals out of the 5V ADC unidirectional or bidirectional?
• On the left side of the schematic, the digital signals are for input to the 1.8V MCU

The ADC should be placed before ISO7742 and have the digital output signals isolated as shown in the image below:




Thank you,
Manuel Chavez

Hi Manuel,

Once again thanks for the response.

1. Its a individual 5V analog channels, Not differential.

2. Signals are Unidirectional. they are directly taken from the 5V sensors, where the sensors will output 0-5V analog values.

Hi Manjunath,

Of course; I'm glad to help.

1. Its a individual 5V analog channels, Not differential.

Noted, thank you

2. Signals are Unidirectional. they are directly taken from the 5V sensors, where the sensors will output 0-5V analog values.

Noted, thank you


Which ADC part number will be used in this system? This will be help identify the structure of its digital communication lines for isolation.


Respectfully,
Manuel Chavez

Hi Manuel,

I too don't have any idea on the ADC device, Our design is a customized PCB with Main LTE cAT M1 technology,

So just giving an option, Any sensor data can be connected to these lines.

Thanks

Manjunath

Hi Manjunath,

What is the frequency of the sensor data? What is the desired resolution of digital signals?

Recommending an ADC may be out of the help I can provide, but I can refer you to the appropriate team. Additionally, if the ADC to MCU communication protocol is known (like SPI), I can help select the appropriate digital isolator.

Please let me know if this has been determined.


Thank you, and have a great weekend,
Manuel Chavez

Hi Manuel,

Thanks,

ADC resolution is 12 bit, don't know about frequency of the sensor data right now,

But we are not using any ADC communication protocol(Like SPI), Its only a single channel analog data which will vary 0-5V voltage levels. thats all.

considering that, please find the attached circuit and review the same, I need a isolation first and then I'm using a translator for level shifting, Will this works?

One more question, If my analog signal voltage level at the input to the ISO7742 is 3V will I get same voltage at its output?

Regards

Manjunath

Hi Manjunath,

Thank you for the information. Digital isolators are only capable of isolating binary (digital or HIGH/LOW) data. If an analog signal is input to the isolator, the output will be either Vcc (HIGH) or GND (LOW) based on internal thresholds.

Because of this, an ADC is necessary between the analog signal and the digital isolator, regardless of which communication protocol it uses. The protocol affects how many digital isolation channels are needed and their forward/reverse configuration.

The digital isolator output level will be either the local GND or Vcc potentials to represent digital LOW and HIGH respectively.


Best,
Manuel Chavez

Yes Manuel Understood,

Then Can you please suggest me how to isolate an individual analog lines?

Hi Manjunath,

An option to consider is using the ISO122 isolated amplifier device. This device requires 5V power supplies on the isolated and non-isolated sides, however an attenuation circuit may be used on the isolated output to reduce the signal to a lower scale, like 1.8V, as shown below:


Is this a possibility in your system?


Thank you,
Manuel Chavez

Hi Manuel,

Yeah correct, but what is attenuation circuit we should use here,

Please mention that also, to confirm the circuit.

regards,

Manjunath

request you to share the complete circuit blocks.

Thanks

Manjunath

Hi Manjunath,

Please allow me a until Monday or Tuesday next week to share these circuit blocks.


Thank you for your time,
Manuel Chavez

Okay Manuel.

Regards.

Manjunath

Hi manuel,

Waiting for your reply today.

Hi Manuel,

Thanks for the reply.

The circuit you have shared is for mainly AC signals right?

My requirement is to measure the DC voltage.

For your reference : Below is the sensor Spec that we will be using.

"Sensor Type : Soil moisture sensor.

Distance b/ Senosr and the PCB : 5meters

Digital output interface : Analog value 0V-5V, Single ended

Accuracy : +-4% "

If above circuit can be used for DC also, then after the op-amp I should use a voltage divider to convert the 5V level to 1.8V right?

Hi Manjunath,

The ISO224 input is optimized for sensing signals and I do not expect any incompatabilities. Since ISO224 also has an internal 1/3 V/V gain, an additional attenuator is not necessary. The output voltage marked "Vout" will range from 0V - ~1.7V.

Is it possible for you to confirm this solution through testing?


Thank you,
Manuel Chavez

Hi Manuel,

Please find the attached schematic and review the same.

How u r expecting the test procedure?

Thanks

Manjunath