Performance and reliability are driving many lab and field science (LFS) device and equipment applications. However, bill-of-material (BOM) inefficiencies can increase costs and reduce measurement precision. These inefficiencies are often related to sourcing of signal chain devices. Access to a complete signal chain solution with knowledgeable and experience application support from a single trusted supplier can enable system and design efficiencies and rapid time-to-market, which is otherwise challenging to achieve. A reference design from Analog Devices’ “Circuits From The Lab”, a Dual-channel Colorimeter with Programmable Gain transimpedance amplifiers and digital synchronous detection, is used to demonstrate the benefits of a complete signal chain solution from a single supplier.
As expanding market opportunities for producing quality lab and field science (LFS) devices and equipment increases, the pressure to bring these solutions to market faster is also growing. Achieving these rapid design cycles, while maintaining precision and reliability of the overall device, relies heavily on the quality sourcing efficiency of the critical signal chain components. Highly integrated signal chain components, such as amplifiers and analog-to-digital converters (ADCs), can lead to improved bill-of-material (BOM) efficiency and lower costs. Additionally, being able to source the critical signal chain components from a single supplier can bring many cost benefits and open opportunities for enhanced application support. Ultimately, these factors could reduce the burden on design resources and can lead to a better performing design in a fraction of the time.
Caption: The Zedboard IOT Kit is used to generate a modulated signal and interface for spectroscopy applications with the Dual Channel Colorimeter evaluation board, CN0363, from Analog Devices.
Quality Reference Designs Offer Circuit Solutions Adaptable To Many Applications
Analog Devices offers a large library of reference designs freely available on their website. These reference designs can be used out of the box as circuit solutions, or the detailed explanations can aid a designer in adapting the circuits to their own application. These reference designs feature a broad range of components that could be incorporated into precision laboratory or field test and measurement or service equipment.
For example, Circuit Note 0363 (CN0363), a Dual-Channel Colorimeter With Programmable Gain Transimpedance Amplifiers and Digital Synchronous Detection is built with leading ADC and precision amplifier solutions from Analog Devices. There is a video that features the Circuit Note and provides insights into the process along with this article.
Click here to watch the video.
FPGA Code of the CN0363 is also available through Analog Devices.
The circuit performs absorption spectroscopy measurements with three different wavelengths of light, leveraging a circuit typology that reject noise sources for a very accurate result. Such devices are commonly used for chemical analysis and environmental monitoring devices and instruments for use in determining the characteristics and concentrations of liquid samples.
Caption: The key signal chain components of the Evaluation Board CN0363, ADA4528-1, ADA4805, and AD7175-2, are used as transimpedance amplifiers, high slew-rate buffer amplifiers, and a high precision analog-to-digital converter, respectively.
Key Signal Chain Components Reduce BOM Complexity And Enhance Performance
The Dual-Channel Colorimeter is comprised of a three frequency modulated light source transmitter that sends light through a liquid sample, and is attenuated by the frequency characteristics of the sample before being received by their respective photodiodes. The ADA4528-1 is used as a transimpedance amplifier for generating a voltage from the current output of the photodiode receivers. As the current output of the photodiodes may be very weak for highly absorptive liquids, the ADA4528-1 auto-zero amplifier features are leveraged to eliminate offset error, 1/f noise, and also induce very low broadband noise. To avoid noise spikes at the auto-zero frequency, the auto-zero frequency is at roughly 200 kHz, which is well beyond the 3dB signal bandwidth of the amplifier.
Caption: The two separate resistor feedback networks of the transimpedance amplifier enable configuration of the transimpedance amplifier to optimize the output for materials of higher and lower absorption characteristics.
Though the ADA4528-1 may have a very low offset voltage, the offset may limit rail-to-rail operation if the offset is negative. So as not to require a negative voltage supply and to ensure the transimpedance amplifiers do not clip, the ADA4805-1 is used to supply a buffered offset bias voltage of 100mV to the anode of the photodiode. The ADA4805-1 op amp is used as a highly stable voltage buffer capable of driving significant capacitive loads, and is well suited for decoupling. The bias voltage of the photodiodes and the output of the AD5201 digital potentiometer used to set the LED current are buffered with the ADA4805-1 op amp.
A beam splitter is used to split the red, green, and blue (RGB) LED outputs to the sample and a reference receiver. Both the sample receiver and reference receiver comprise identical programmable gain transimpedance amplifier circuits that output to separate channels of the ultralow noise 24-bit Σ-Δ ADC, the AD7175-2. In order to sample both channels within a single-cycle settling time, the ADC is configured to output a data rate of 250 kSPS with a sinc5+sinc1 filter. This method produces an effective sampling rate, per channel, of 25 kSPS and an output value every 40 µs.
Caption: The SPI output of the AD7175-2 enables 250 kSPS of reference and sampled data, which is used to generate IQ data and detailed digital comparison of the data.
To prevent frequencies above 12.5 kHz, such as the odd harmonics of square wave modulation, from aliasing in the passband of the ADC, a synchronous demodulation stage is implemented with an FPGA. Nevertheless, if a noise source does exist at the modulation frequencies, it will be folded back to the fundamental frequencies. Avoiding this occurrence requires maintaining a modulation and sample frequency relationship dictated by the equation:
Signal Chain Benefits Of Ultralow Noise, Zero-Drift, And Rail-to-Rail Input/Output Op Amp
The ADA4528-1 is designed to fit applications where limiting error in the signal chain is of absolute importance. This can include many sensor and medical applications, in which small voltages are sampled and converted. The amplifier demonstrates a typically low noise of 5.6 nV/√Hz at f = 1 kHz and 97 nV p-p at f = 0.1 Hz to 10 Hz at AV of +100, and a very low offset drift of 0.015 μV/°C with a maximum offset voltage of 2.5 μV. These specifications are made over the extended industrial and automotive temperature range of −40°C to +125°C.
Capable of rail-to-rail input and output (RRIO) with a wide operating voltage supply range of 2.2 V to 5.5V or dual-supply operation of +/- 1.1 V to +/- 2.75 V, the ADA4528-1 also offers a high CMRR of 135 dB minimum and PSRR of 130 dB minimum. These features are especially important when dealing with very low level signals near the noise floor. A high gain of 130 dB and unity-gain crossover of 4 MHz enables a high gain bandwidth product of 3 MHz at AV = +100 and a -3dB closed-loop bandwidth of 6.2 MHz.
The ADA4528-1 is available in 8-lead MSOP and 8-lead LFCSP packages. More information on this component and the dual package, ADA4528-2, can be found in Application Note AN-1114, “Lowest Noise Zero-Drift Amplifier Has 5.6 nV/√Hz Voltage Noise Density.”
Low Thermal Offset Drift, Power, and Noise Rail-to-Rail Op Amp Provides Enhanced Slew Rate Features
The ADA4805-1 provides high performance with enhanced power saving features, specifically designed for high resolution data conversion systems at low power levels. Ideal applications also include battery-powered instrumentation, portable point of sale terminals, and micropower active filters. With a low quiescent current of 495 μA, the ADA4805-1 also includes a shutdown pin function, while in shutdown, further lowers the quiescent supply current to 3 μA. In order to enable rapid analog-to-digital conversion (ADC) functions, the boot up time of the amplifier from shutdown to fully on, is only 3 μs.
Though the ADA4805-1 operates with very low power, the device also features a very high -3dB bandwidth of 105 MHz with a slew rate of 160V/μs and a settling time to 0.1% of 35ns. The high slew rate and fast settling time allow the ADA4805-1 to be leveraged as a large capacitive load driving buffer amplifier with excellent decoupling characteristics.
Also, the maximum input offset voltage of 125 μV and a typical input offset drift of 0.2 µV/°C are exhibited for a wide supply range of 3 V to 10 V at +/- 1.5 V, 3 V, 5 V, or +/- 5 V. Though the input features a common-mode range of -Vs - 0.1 V to +Vs - 1V, the output features a low noise of 5.9 nV/√Hz at 100 kHz and 0.6 pA/√Hz at 100 kHz and low distortion of −102 dBc/−126 dBc HD2/HD3 at 100 kHz for complete rail-to-rail output.
In order to enable the lowest noise performance over the extended industrial and automotive temperature range of −40°C to +125°C, the ADA4805-1 amplifier is built with Analog Devices, Inc., proprietary extra fast complementary bipolar (XFCB) process and is offered in very small 6-lead SOT-23 and 6-lead SC70 packages. A dual amplifier version, the ADA4805-2, is also available in 8-lead MSOP and 10-lead LFCSP packages.
Σ-Δ ADCs With Low Noise And Fast Settling Features Offer Flexible Features For Laboratory And Field Science Applications
The AD7175-2 offers a very low noise and fast settling Σ-Δ ADC with noise free bits of 17.2 at 250 kSPS, 20 at 2.5 kSPS, and 24 at 20 SPS with an INL of +/-1 ppm of the full scale range. The ADC can be configured as a 2-channel fully differential or 4-channel pseudo differential converter with a flexible output rate of 5 SPS to 250 kSPS. The inputs from the internal cross channel multiplexer are passed through rail-to-rail analog input buffers, which present a high impedance input.
Several powering options are available for the ADC, at 5 V AVDD1 or +/-2.5 V AVDD1/AVSS and 2 V to 5 V AVDD2 and IOVDD, and only a typical current draw of 8.4 mA. The ADC also includes an on-chip reference voltage of 2.5 V and low drift of +/- 2ppm/°C.
Conveniently, the AD7175-2 features integrated analog and digital signal conditioning blocks that enable each analog input channel to be independently configured. For example, the 50 Hz or 60 Hz rejection of 85dB with 50 ms settling time at a 27.27 SPS output data rate, is a function provided by digital filters conditioning the input signal. Additionally, other digital processing functions, such as offset and gain calibration, are also configurable independently per channel. The GPIO and MUX I/O control digital inputs can be used to control the input multiplexer, or the AD7175-2 can automatically cycle between the inputs, lowering the control circuitry complexity.
With a flexible 3- or 4-wire Schmitt trigger or SCLK digital interface, the ADC is capable of SPI, QSPI, MICROWIRE, and DSP compatible data exchange. The external or internal clock oscillator can be used for timing, and the ADC offers internal clock output for reference. These features, and the extended industrial temperature range of −40°C to +105°C, lend the AD7175-2 to process control, medical/scientific multichannel instrument, temperature/pressure, and chromatography instrument applications. The ADC is delivered in a 24-lead TSSOP package.
Caption: The AD7175-2 features many enhanced digital control features and automated functions, which reduce the number of external components required.
Conclusion
With the Circuit Note CN0363, Analog Devices is able to demonstrate the benefits of circuit solutions supplier with a diverse enough product line to supply a virtually complete signal chain solution for Lab and Field Science and other applications. These benefits help to reduce BOM complexity and costs, while enabling reduced time-to-market and ease of design for system design engineers.