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Part Numbers with "Z" indicate RoHS Compliance. Boards checked are needed to evaluate this circuit.
- EVAL-CN0365-PMDZ ($350.00) 16-Bit, 600 kSPS, Low Power Data Acquisition System for High Temperature Environments
- EVAL-SDP-CB1Z ($108.90) Eval Control Board
- SDP-PMD-IB1Z ($60.50) PMOD to SDP Interposer Board w/Power Supply
Features & Benefits
- 16-Bit, 600kSPS Data Acquisition System
- System guaranteed at +175°C
- Low Power
Markets and Technologies
Technical Articles (2)
Circuit Function & Benefits
A growing number of applications require data acquisition systems that must operate reliably at very high ambient temperature environments, such as downhole oil and gas drilling, avionics, and automotive. The circuit shown in Figure 1 is a 16-bit, 600 kSPS successive approximation analog-to-digital converter (ADC) system using devices rated, characterized, and guaranteed at 175°C. Because many of these harsh environment applications are battery-powered, the signal chain has been designed for low power consumption while still maintaining high performance.
This circuit uses the AD7981, a low power (4.65 mW at 600 kSPS), high temperature PulSAR® ADC, driven directly from the AD8634 high temperature, low power op amp. The AD7981 ADC requires an external voltage reference between 2.4 V and 5.1 V, and in this application, the voltage reference chosen is the micropower ADR225 precision 2.5 V reference, which is also qualified for high temperature operation and has a very low quiescent current of 60 μA maximum at 210°C.
All of the ICs in this design have packaging specially designed for high temperature environments, including monometallic wire bonds. In addition, this reference design describes the selection of the passive components, printed circuit board PCB) materials, and construction techniques to enable operation at these extreme temperatures. A complete design support package including bill of materials, schematic, assembly, and layout files is also available.
Analog to Digital Converter
The heart of this circuit is the AD7981, a 16-bit, low power, single supply ADC that uses a successive approximation architecture (SAR) and is capable of sampling up to 600 kSPS. As shown in Figure 1, the AD7981 uses two power supply pins: a core supply, VDD, and a digital input/output interface supply, VIO. The VIO pin allows a direct interface with any logic between 1.8 V and 5.0 V. The VDD and VIO pins can also be tied together to save on the number of supplies needed in the system, and they are independent of power supply sequencing.
The AD7981 powers down automatically between conversions to save power. Therefore, the power consumption scales linearly with the sampling rate, making the ADC well suited for both high and low sampling rates (even as low as a few Hz) and enables very low power consumption for battery-powered systems. Additionally, oversampling techniques can be used to increase the effective resolution for low speed signals.
The AD7981 has a pseudo differential analog input structure that samples the true differential signal between the IN+ and IN− inputs and rejects the signals common to both inputs. The IN+ input can accept the unipolar, single-ended input signal from 0 V to VREF, and the IN− input has a restricted range of GND to 100 mV. The pseudo differential input of AD7981 simplifies the ADC driver requirement and lowers power dissipation. The AD7981 is available in a 10-lead MSOP rated for 175°C. A simplified connection diagram is shown in Figure 2.
The input of AD7981 can be driven directly from low impedance sources; however, high source impedances significantly degrade the ac performance, especially total harmonic distortion (THD). Therefore, it recommended to use an ADC driver or op amp, such as the AD8634, to drive the input of the AD7981 as shown in Figure 3. At the start of the acquisition time, the switch closes, and the capacitive DAC injects a voltage glitch (kickback) on the ADC input. The ADC driver helps to settle this kickback as well as to isolate it from the signal source.
The low power (1.3 mA/amplifier) AD8634 dual precision op amp is suited for this task because its excellent dc and ac specifications are a good fit for sensor signal conditioning and elsewhere in the signal chain. While the AD8634 has rail-to-rail outputs, the input requires 300 mV headroom from the positive and negative rails.
This headroom requirement necessitates the negative supply, which was chosen to be −2.5 V.
The AD8634 is available in an 8-lead SOIC rated for 175°C and an 8-lead FLATPACK rated for 210°C.
The RC filter between the ADC driver and AD7981 attenuates the kickback injected at the input of the AD7981 and band limits the noise coming to its input. However, too much band limiting can increase settling time and distortion. The calculation of the optimum RC value is primarily based on the input frequency and throughput rate. For the example shown, R = 85 Ω and C = 2.7 nF are the optimum values yielding a cutoff frequency of 693 kHz. Details of the calculations can be found in the Analog Dialogue article, Front-End Amplifier and RC Filter Design for a Precision SAR Analog-to-Digital Converter.
In this circuit, the ADC driver is in unity gain buffer configuration. Adding gain to the ADC driver reduces the bandwidth of the driver and lengthens the settling time. In this case, the throughput of the ADC may need to be reduced or an additional buffer as a driver can be used after the gain stage.
The ADR225 2.5 V voltage reference uses only 60 μA maximum of quiescent current at 210°C and has very low drift of 40 ppm/°C typical, making it an ideal device for this low power data acquisition circuit. The ADR225 has an initial accuracy of ±0.4% and can operate over a wide supply range of 3.3 V to 16 V.
The voltage reference input of the AD7981, like other SAR ADCs, has a dynamic input impedance and must therefore be driven by a low impedance source with efficient decoupling between the REF pin and GND, as shown in Figure 4. The AD8634 is well suited as a reference buffer in addition to its ADC driver application.
Another advantage of using a reference buffer is that the noise on the voltage reference output can be further reduced by adding a low pass RC filter. In this circuit, a 49.9 Ω resistor and 47 μF capacitor give a cutoff frequency of approximately 67 Hz.
During conversions, current spikes as high as 2.5 mA can occur on the AD7981 reference input. A high value reservoir capacitor is placed as close as possible to the reference input to supply that current and to keep the reference input noise low. Typically, a low ESR, 10 μF or more, ceramic capacitor is used; however, for high temperature applications, ceramic capacitors are not available. For this reason, a low ESR, 47 μF tantalum capacitor was chosen that has minimal impact on the performance of the circuit.
The AD7981 offers a flexible serial digital interface compatible with SPI, QSPI, and other digital hosts. The interface can be configured for a simple 3-wire mode for the lowest input/output count, or 4-wire mode that allows options for the daisy-chained readback and busy indication. The 4-wire mode also allows independent readback timing from the CNV (convert input), which enables simultaneous sampling with multiple converters.
The PMOD interface used on this reference design implements the simple 3-wire mode with SDI tied high to VIO. The VIO voltage is supplied externally from the SDP-PMOD interposer board.
This reference design requires external, low noise power supplies for the +5 V and −2.5 V rails. Because the AD7981 is low power, it can be supplied directly from the reference buffer, as shown in Figure 5, thereby eliminating the need for an additional power supply rail, saving power and board space.
IC Packaging and Reliability
Devices in the Analog Devices high temperature portfolio go through a special process flow that includes design, characterization, reliability qualification, and production test. Part of this process is special packaging designed specifically for extreme temperatures. A special material is used for the 175°C plastic packages in this circuit.
One of the major failure mechanisms in high temperature packaging is the bond wire-to-bond pad interface, particularly when gold (Au) and aluminum (Al) metals are mixed, as is typical in plastic packages. Elevated temperature accelerates the growth of AuAl intermetallic compounds. It is these intermetallics that are associated with bond failures, such as brittle bonds and voiding, which can occur in a few hundred hours, as shown in Figure 6.
To avoid these failures, Analog Devices uses an over pad metallization (OPM) process to create a gold bond pad surface for the gold bond wire to attach. This monometallic system does not form intermetallics and has been proven reliable in qualification testing with over 6000 hours soak at 195°C, as shown in Figure 7.Au Ball Bond on Al Pad, After 500 Hours at 195°CC, as shown in Figure 7.
Although Analog Devices has shown reliable bonding at 195°C, the plastic package is rated for operation only up to 175°C due to the glass transition temperature of the molding compound.
In addition to the 175°C rated products used on this circuit, 210°C rated models are also available in a ceramic FLATPACK package. Known good die (KGD) are also available for systems that require custom packaging. Analog Devices has a comprehensive reliability qualification program for high temperature products that includes high temperature operating life (HTOL), with the devices biased at the maximum operating temperature. High temperature products are data sheet specified for a minimum of 1000 hours at the maximum rated temperature. Full production testing is the last step required to guarantee performance for each device that is manufactured. Each device in the Analog Devices high temperature portfolio is production tested at elevated temperature to ensure that performance is met.
Analog Devices has a comprehensive reliability qualification program for high temperature products that includes high temperature operating life (HTOL), with the devices biased at the maximum operating temperature. High temperature products are data sheet specified for a minimum of 1000 hours at the maximum rated temperature. Full production testing is the last step required to guarantee performance for each device that is manufactured. Each device in the Analog Devices high temperature portfolio is production tested at elevated temperature to ensure that performance is met.
Passive components chosen must be rated for high temperatures. For this design, 175°C+ thin film, low TCR resistors were used. COG/NPO capacitors were used for low value filter and decoupling applications and have a very flat coefficient over temperature. High temperature rated tantalum capacitors are available in larger values than ceramic capacitors and are commonly used for power supply filtering. The SMA connector used on this board is rated for 165°C; therefore, it must be removed for long duration testing at elevated temperatures. Similarly, the insulation material on the 0.1” header connectors (J2 and P3) is only rated for short durations at high temperature and must also be removed for prolonged high temperature testing.
PCB Layout and Assembly
The PCB for this circuit is designed so that the analog signals and digital interface are on opposites sides of the ADC, with no switching signals running under the IC or near analog signal paths. This design minimizes the amount of noise that is coupled into the ADC die and supporting analog signal chain. The pinout of the AD7981, with all its analog signals on the left side and all its digital signals on the right side, eases this task. The voltage reference input, REF, has a dynamic input impedance and must be decoupled with minimal parasitic inductances, which is achieved by placing the reference decoupling capacitor as close as possible to the REF and GND pin and making the connection to the pin with a wide, low impedance trace. The layout of this board was purposely designed with components only on the top side of the board, to facilitate testing over temperature where heat is applied from the bottom of the board. For further layout recommendations, see the AD7981 data sheet.
For high temperature circuits, special circuit materials and assembly techniques must be used to ensure reliability. FR4 is a common material used for PCB laminates; however, commercial grade FR4 has a typical glass transition temperature of approximately 140°C. Above 140°C, the PCB begins to break down, delaminate, and cause stress on components. A widely used alternative for high temperature assemblies is polyimide, which typically has a glass transition temperature of greater than 240°C. A 4-layer polyimide PCB was used in this design.
The PCB surface is also a concern, especially when used with solders containing tin, because of the tendency of the solder to form intermetallics with copper traces. A nickel-gold surface finish is commonly used, where the nickel provides a barrier, and the gold provides a good surface for the solder joint bonding. High melting point solder must also be used with a good margin between the melting point and maximum operating temperature of the system. SAC305 lead free solder was chosen for this assembly. With a melting point of 217°C, there is a margin of 42°C from the highest operational temperature of 175°C.
The AD7981 is specified for 91 dB SNR typical with a 1 kHz input tone and a 5 V reference. However, when using low reference voltages, as is common in low power/low voltage systems, some degradation in SNR is expected. From the AD7981 data sheet typical performance curves, approximately 86 dB of SNR is expected at room temperature with a 2.5 V reference. This SNR value compares well with the performance achieved when the circuit was tested at room temperature with approximately 86 dB SNR, as shown in Figure 8.
When this circuit is evaluated over temperature, SNR performance only degrades to approximately 84 dB at 175°C, as shown in Figure 9. THD remains better than −100 dB, as shown in Figure 10. The FFT summary for the circuit at 175°C is shown in Figure 11.
Circuit Evaluation & Test
This circuit uses the EVAL-CN0365-PMDZ circuit board, the SDP-PMD-IB1Z interposer board, and the EVAL-SDP-CB1Z system demonstration platform (SDP) board. The interposer board and the SDP board have 120-pin mating connectors. The interposer board and the EVAL-CN0365-PMDZ board have 12-pin PMOD matching connectors, allowing quick setup and evaluation of the circuit performance. The EVAL-CN0365-PMDZ board contains the circuit to be evaluated, as described in the CN-0365, and the SDP evaluation board is used with the CN-0365 Evaluation Software.
The following equipment is needed:
- EVAL-CN0365-PMDZ board
- System Demonstration Platform (EVAL-SDP-CB1Z)
- PMOD to SDP Interposer Board (SDP-PMD-IB1Z)
- CN-0365 Evaluation Software
- Function generator/signal source, such as the Audio Precision SYS-2522 used in these tests
- Power supplies: +5 V and −2.5 V
- Power supply: +6 V wall wart (EVAL-CFTL-6V-PWRZ)
- PC operating Windows® XP (SP2), Windows Vista or Windows 7 Business/Enterprise/Ultimate editions (32-bit/64-bit systems) with USB port and USB cable
To get started, take the following steps:
- Download the CN-0365 Evaluation Software to the PC from ftp://ftp.analog.com/pub/cftl/CN0365.
- Install the software prior to connecting the SDP board to the USB port of the PC, to ensure that the SDP board is recognized when it connects to the PC.
- Unzip the downloaded file.
- Run the setup.exe file.
- Follow the on-screen prompts to finish the installation. It is recommended to install all software components to the default locations.
Functional Block Diagram
Figure 12 shows the functional diagram of the test setup.
To set up the circuit, take the following steps:
- Connect the EVAL-CFTL-6V-PWRZ (+6 V dc power supply) to the SDP-PMD-IB1Z interposer board via the dc barrel jack.
- Connect the SDP-PMD-IB1Z interposer board to the EVAL-SDP-CB1Z SDP board via the 120-pin CON A connector.
- Connect the EVAL-SDP-CB1Z SDP board to the PC via the USB cable.
- Connect the EVAL-CN0365-PMDZ evaluation board to the SDP-PMD-IB1Z interposer board via the 12-pin header PMOD connector.
- Connect the +5 V (VS+) and −2.5 V (VS−) power supplies to the EVAL-CN0365-PMDZ P3 header. The VDD voltage (2.5 V) does not need an external connection in the default configuration because it is generated on-board.
- Connect the signal source to the EVAL-CN0365-PMDZ via the SMA connector.
- Set the Audio Precision SYS-2522 (or equivalent signal generator) for a 1 kHz frequency and a 2.5 V p-p sine wave with a 1.25 V dc offset.
Launch the evaluation software. The software communicates to the SDP board if the Analog Devices System Development Platform drivers are listed in Windows Device Manager. After USB communications are established, the evaluation software can be used to test, view, and save circuit performance metrics.
Further details on the operation of the software can be found in the UG-340 and at the Evaluating 14-/16-/18-Bit ADCs from the 8/10 LEAD PulSAR® Family wiki page.
For testing over temperature in an environmental chamber,extension harnesses (not supplied) can be used for the analog input, power supply, and PMOD connections. These harnesses must be as short as possible, and best practices must be used to avoid noise. The SMA connector used on this board is rated for 165°C; therefore, it must be removed for long duration testing at elevated temperatures. Similarly, the insulation material on the 0.1” header connectors (J2 and P3) is only rated for short durations at high temperature and must also be removed for prolonged high temperature testing.
A photo of the EVAL-CN0365-PMDZ board is shown in Figure 13.