LMP7721: Confusion about example schematics

BenvanDijk

Part Number: LMP7721
Other Parts Discussed in Thread: LMP7715, LM4140, LMP7716, LMP91200

Dear forum members,

I am designing a pH/ORP interface from -1500 to + 1500 mV with a three wire PT-1000 input.

I did a lot of research last week and decided to use the approach as can be seen in AN-1852 for the pH part of the circuit.

Circuit 1:

However, during the design process I also noticed the following circuit (TI CircuitCookbook ADCs):

Circuit 2:

This circuit uses a similar approach, the only differences are that the second LMP7721 is replaced with a LMP7715 which significantly reduces the cost price of the BOM and an additional LMP7715 with unknown functionality (at least for me):

The following website also designed a PCB according to the CircuitsCookbook ADCs example:

Website: https://www.360customs.de/en/2019/12/orp-redox-ph-elektrometer-shield-fuer-wemos-d1-mini-mit-lmp7721-ads1115-adm3260

PDF Circuit: /cfs-file/__key/communityserver-discussions-components-files/14/ORP_5F00_FRONTENT_5F00_ESP8266.pdf

Questions:

1. Why does circuit 1 use 2x LMP 7721 and circuit 2 only one? I know that this is possible to reduce costs because the LMP7721 is quite expansive, but what is the advantage of using the LMP7721 to generate the offset voltage compare to using the LMP7715?

2. In circuit 2, what is the use of the second LMP7715 and what are the benefits (also compare to circuit 1)

3. Which circuit would you recommend considering a noisy environment (e.g. in a factory with 3-phase big pumps)?

4. Do you think that by selecting the correct voltage regulator (LM4140) a measuring range from -1500 to + 1500 mV can be achieved?

5. The German site uses LMP7716 instead of LMP7715, what can be a reason to select this component (besides the price).

6. Is it ok to upload the schematics so you can give feedback?

Thanks a lot and best regards,

Ben

over 4 years ago

0 Raymond Zhang1 over 4 years ago

TI__Guru 54871 points

Hi JR JR,

Please use the icon circled in red to upload an image file, use the icon circled in green to upload a standard file.

I am going to answer the following questions.

4. The German site uses LMP7716 instead of LMP7715, what can be a reason to select this component.

The application information is listed on p.14 of the datasheet. Postfix in Q is automotive grade. There are some performance and package variations between three parts. It is the same IC die, but certain performance are slightly differently, and it is described throughout the datasheet.

5. Is it ok to upload the schematics so you can give feedback?

This is public forum. Yes, we are able to assist you based on your request.

I am designing a pH/ORP interface from -1500 to + 1500 mV with a three wire PT-1000 input.

PT-1000 is Pt temperature sensor (RTD) with R=1000Ohm at 0C and 138.4Ohm at 1000C. For pH meter application, temperature measurement is used to compensate temperature effects of redox reaction in the following equation. Anyway, I will reply to your questions, after you upload the images.

Best,

Raymond

0 BenvanDijk over 3 years ago in reply to Raymond Zhang1

Prodigy 20 points

Dear Raymond,

Thanks for your prompt reply and noticing about the missing images. I updated the post and included the circuits as you explained, thanks for that.

Based on your reply, I studied the Datasheets a bit more in detail and noticed the reduced current consumption and also the difference in price and package. I will use the LMP7715 for my circuit (or there should be a good reason to also use the LMP7721 in this case).

Best regards,

Ben

0 Raymond Zhang1 over 3 years ago in reply to BenvanDijk

TI__Guru 54871 points

Hi Ben,

Please take a look at the Ibias requirements. I do not know what is your design or application requirements. LMP7721 is designed for high impedance PH electrode. We have other op amps that may meet your pH measurement/application's need and lower the system costs, see below.

https://www.ti.com/amplifier-circuit/op-amps/cross-reference.html?keyMatch=OP%20AMP%20CROSS%20REFERENCE%20TOOLS&tisearch=Search-EN-everything

2. In circuit 2, what is the use of the second LMP7715 and what are the benefits (also compare to circuit 1)

The red rectangular block is the reference voltage to a pH reference electrode (Es).

The green rectangular block is the working electrode in pH-measuring electrode (Ex), see the transfer function of the pH electrode from previous reply.

The blue rectangular block is used to improve common mode rejection of the pH-measurement electrode (reject 50/60Hz noise).

The above design is better design for noisy environment. You may also use LMP91200 in this application, see the link below.

https://www.ti.com/lit/ds/symlink/lmp91200.pdf?ts=1604942872386&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLMP91200

https://www.ti.com/lit/ug/tidua47b/tidua47b.pdf?ts=1604943093328&ref_url=https%253A%252F%252Fwww.google.com%252F

3. Which circuit would you recommend considering a noisy environment (e.g. in a factory with 3-phase big pumps)?

I need to know your how precise your pH measurement (e.g. +/-0.1pH or +/- 0.01pH). In a noisy industrial environment, you will need to shield all cables and have a proper earth ground in your sensors and drivers, which it will reduce noise couplings into the high impedance nodes (behaves like Faraday shield). In an industrial operating environment, noise may be coupled in the following mechanism - capacitive, inductive or radiative.

4. Do you think that by selecting the correct voltage regulator (LM4140) a measuring range from -1500 to + 1500 mV can be achieved?

I am not clear why you need +/-1500mV range. For pH meter, the usable measurement range from acid to base is only +/-414mV. So you may need +/-600mV range or so. The application note is designed for 0.01pH accuracy, and LM4140-1.0V will be adequate.

https://www.ti.com/power-management/voltage-reference/series-voltage-reference/products.html#p241max=2.5;3&sort=p1498;asc

If you have additional questions, please let us know.