LMP2234 and ADS1115 - How to design an ADC driver?

andre.robotokogaku

Other Parts Discussed in Thread: ADS1115, LMP2234

Hello folks,

I'm working in a design with low (uV) analog signals that must be converted to digital. I have already designed the AFE which raise the signal to 100mV - 1V orders of magnitude. After the AFE stage, the circuit has a unity-gain, low-pass filter that serves as an anti-aliasing filter. The AFE + LPF are implemented using a LMP2234 op amp. Then, the signal is fed to the ADC (in this case I'm using ADS1115, 16-bit Sigma-Delta ADC).

My question is if this topology still needs to have a ADC driver, or does the LPF servers as the ADC driver?

Thanks in advance,

André Araujo.

over 12 years ago

0 Luis Chioye over 12 years ago

TI__Guru 54416 points

Hi Andre,

The ADS1115 has an internal PGA that works well with connection to differential sources with moderate low source impedance. I suspect that the LMP2234 in unity gain with low pass filter would be able to drive the device without issues; but if you provide your schematic detail; we will be happy to review it. In many applications, a simple RC low pass filter is used. The general recommendation is to avoid increasing the RC filter resistance beyond 1kOhm because of the interaction that occur with the ADC input impedance. In many applications, a simple RC filter as shown below is used.

Thank you and Best Regards,

Luis

0 andre.robotokogaku over 12 years ago in reply to Luis Chioye

Intellectual 440 points

Hello Luis,

Thanks for the reply!

In fact, I designed an active second order low pass filter (Butterworth, using Sallen Key topology). The cutoff frequency is around 3 Hz, as I plan to convert at 10Hz. Here's the schematic:

Pin 8 (U6) goes straight to one of the ADC's inputs. Do you think the circuit needs an additional op-amp to drive the ADC?

Thanks again,

André.

0 Luis Chioye over 12 years ago in reply to andre.robotokogaku

TI__Guru 54416 points

Hi Andre,

I don't see an issue with the amplifier stage above.

You coud also use the FilterPro tool to assist with the filter design. Please find the link below.

http://www.ti.com/tool/filterpro

The filter pro tool may help you design the filter according to your needs. Two possible configurations, Sallen Key and Multiple feedback configurations are below. The idealized response is also shown.

Best Regards,

Luis

Sallen Key

Multiple Feedback

0 andre.robotokogaku over 12 years ago in reply to Luis Chioye

Intellectual 440 points

Hi Luis,

Thank you very much for the reply. Indeed, I use Filter Pro to ease the design of filters, as well as other tools. I like very much this website: http://sim.okawa-denshi.jp/en/, which has several tools to design filters, comparators, amplifiers and stuff like that. This link is useful when designing LPFs: http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm.

Do you recommend any further reading about ADC drivers? I'm still confused about it's necessity, unless you need to convert a single-ended signal into differential. Both SAR and Sigma-Delta ADCs have the same input requirements? What happens in an ADC input pin that leads to the need of a special amplifier? Or a simpler question: what differs an ADC driver from a regular Op Amp?

I searched Texas and Analog Devices but still haven't found clear answers.

Thanks again,

André Araujo.

0 Tom Hendrick over 12 years ago in reply to andre.robotokogaku

TI__Guru**** 174576 points

Hi Andre,

Thank you for the links! Here are a few additional resources for you:

Designing SAR ADC drive circuitry, Part I: A detailed look at SAR ADC operation. Analog Zone. Download at: http://www.analogzone.com/acqt0221.pdf

Designing SAR ADC drive circuitry, Part II: Input behavior of SAR ADCs. Analog Zone. Download at: http://www.analogzone.com/acqt1003.pdf

Designing SAR ADC drive circuitry, Part III: Designing the optimal input drive circuit for SAR ADCs. Analog Zone. Download at: http://www.analogzone.com/acqt0312.pdf

0 andre.robotokogaku over 12 years ago in reply to Tom Hendrick

Intellectual 440 points

Hello Tom,

Thank you very much for the references! Although these links discuss aspects of SAR ADCs , they are very interesting.

I'll try to remember going back here if I found anything specific to Sigma-Delta converters.

Best regards,

André Araujo.

0 andre.robotokogaku over 12 years ago in reply to andre.robotokogaku

Intellectual 440 points

Hello again folks,

I have found another discussion topic (Anti-Aliasing Filter for Delta Sigma Converters), which suggests the reading of the Application Report SBAA111 - Understanding the ADC Input on the MSC12xx . It is a very clear description of a Sigma-Delta converter Input and how it works.

Best regards,

André Araujo.

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LMP2234 and ADS1115 - How to design an ADC driver?