Analog to Digital Converter

At the heart of this circuit is the AD7981, a 16-bit, low-power, single-supply ADC that uses a successive approximation architecture and supports sampling rates up to 600 kSPS. As shown in Figure 1, the AD7981 uses two power supply pins: core power (VDD) and digital input/output interface power (VIO). The VIO pins can interface directly with any logic from 1.8 V to 5.0 V. The VDD and VIO pins can also be tied together to save the amount of power required by the system, and they are independent of power supply timing.

Between conversions, the AD7981 automatically shuts down to save power. Therefore, the power consumption is linearly proportional to the sampling rate, making the ADC suitable for both high and low sampling rates (even as low as several Hz), and can achieve very low power consumption and support battery-powered systems. Additionally, oversampling techniques can be used to increase the effective resolution of 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 signals common to both inputs. The IN+ input supports unipolar, single-ended input signals from 0 V to V REF , and the IN− input has a limited range from GND to 100 mV. The AD7981's pseudo-differential inputs simplify ADC driver requirements and reduce power consumption. The AD7981 is available in a 10-pin MSOP package and is specified to 175°C. Figure 2 shows the connection diagram.

ADC driver

The input of the AD7981 can be driven directly from a low-impedance signal source; however, high source impedance can significantly degrade performance, especially total harmonic distortion (THD). Therefore, it is recommended to use an ADC driver or operational amplifier (such as the AD8634) to drive the AD7981 input, as shown in Figure 3. At the beginning of the acquisition time, the switch is closed and the capacitive DAC injects a voltage glitch (kickback) at the ADC input. The ADC driver helps this kickback settle and isolate it from the signal source.

The low-power (1.3 mA/amp) dual-channel precision op amp AD8634 is suitable for this task because its excellent DC and AC characteristics are very beneficial for sensor signal conditioning and other parts of the signal chain. Although the AD8634 has a rail-to-rail output, the input requires 300 mV headroom from the positive supply rail to the negative supply rail.

This headroom requirement necessitates a negative supply, which is chosen to be −2.5 V.

The AD8634 is available in an 8-pin SOIC package rated at 175°C and in an 8-pin FLATPACK package rated at 210°C.

The RC filter between the ADC driver and the AD7981 attenuates the kickback injected at the input of the AD7981 and limits the bandwidth of noise entering this input. However, excessive band limiting may increase settling time and distortion. Calculation of the optimal RC value is primarily based on input frequency and throughput rate. For the example shown, R = 85 and C = 2.7 nF are optimal, resulting in a cutoff frequency of 693 kHz. For detailed calculations, see Analog Dialogue article: Front-end amplifier and RC filter design for precision SAR analog-to-digital converter .

In this circuit, the ADC driver is configured as a unity gain buffer. Increasing the ADC driver gain reduces driver bandwidth and increases settling time. In this case it may be necessary to reduce the ADC throughput rate or use a buffer as a driver after the gain stage.

reference voltage source

The ADR225 2.5 V reference consumes only 60 A maximum quiescent current at 210°C and has an ultra-low drift of 40 ppm/°C typical, making it ideal for use in this low-power data acquisition circuit. The ADR225 has an initial accuracy of ±0.4% and operates over a wide supply range of 3.3 V to 16 V.

Like other SAR ADCs, the AD7981's reference input has dynamic input impedance and must be driven from a low-impedance source with effective decoupling between the REF pin and GND, as shown in Figure 4. In addition to ADC driver applications, the AD8634 is also suitable for use as a reference voltage buffer.

Another benefit of using a reference buffer is that the noise at the reference output can be further reduced by adding a low-pass RC filter. In this circuit, the 49.9 Ω resistor and 47 Ω capacitor provide a cutoff frequency of approximately 67 Hz.

During conversion, a current spike of up to 2.5 mA may occur on the AD7981 reference input. Place a large value storage capacitor as close as possible to the reference input to provide this current and keep noise at the reference input low. Typically low ESR, 10 ¬F or larger ceramic capacitors are used, but for high temperature applications, no ceramic capacitors are available. Therefore, choose a low ESR, 47 ¬F tantalum capacitor, which will have minimal impact on circuit performance.

digital interface

The AD7981 provides a flexible serial digital interface compatible with SPI, QSPI, and other digital hosts. The interface can be configured in either simple 3-wire mode for minimal input/output count, or 4-wire mode to provide daisy chain readback and busy indication options. 4-wire mode also supports independent readback timing of CNV (conversion input), allowing multiple converters to sample simultaneously.

The PMOD interface used in this reference design implements a simple 3-wire mode, and SDI is connected to high-level VIO. The VIO voltage is provided externally by the SDP-PMOD adapter board.

power supply

This reference design requires external low-noise power supplies for the +5 V and −2.5 V supply rails. The AD7981 is a low-power device that can be powered directly from the reference voltage buffer, as shown in Figure 5. This eliminates the need for additional supply rails, saving power and board space.

IC packaging and reliability

Devices in Analog Devices' high-temperature family undergo a special process flow that includes design, characterization, reliability qualification, and production testing. Designing special packages specifically for extreme temperatures is part of the process. The 175°C plastic package in this circuit uses a special material.

A major failure mechanism in high-temperature packages is bond wire-pad interface failure, especially when gold (Au) and aluminum (Al) are mixed (as is often the case with plastic packages). High temperature will accelerate the growth of AuAl intermetallic compounds. It is these intermetallic compounds that cause welding failures, such as brittle welds and voids, which may occur after hundreds of hours, as shown in Figure 6.

To avoid failure, Analog Devices uses a pad metallization (OPM) process to create a gold pad surface for gold wire connections. This monometallic system does not form intermetallic compounds and has been proven to be highly reliable through immersion certification testing at 195°C for 6000 hours, as shown in Figure 7

Figure 7. Gold ball bonding on OPM pad after 6000 hours at 195°C

Although Analog Devices has proven that the soldering is still reliable at 195°C, the plastic package is only rated for a maximum operating temperature of 175°C due to the glass transition temperature of the plastic packaging material.

In addition to the 175°C rated product used in this circuit, a 210°C rated version is available in a ceramic FLATPACK package. Known Good Die (KGD) is also available for systems requiring custom packaging.

For high-temperature products, Analog Devices has a comprehensive reliability certification program that includes high-temperature operating life (HTOL) for devices biased at their maximum operating temperature. The data sheet stipulates that high-temperature products can operate for a minimum of 1,000 hours at the highest rated temperature. Full production testing is the final step to ensure the performance of each device. Every device in ADI’s high temperature family is production tested at high temperatures to ensure performance requirements are met.

Passive components

Passive components that are resistant to high temperatures must be selected. This design uses thin film type low TCR resistors above 175°C. COG/NPO capacitors are used in low value filter and decoupling applications and have a very flat temperature coefficient. High temperature resistant tantalum capacitors have a larger capacitance than ceramic capacitors and are often used for power supply filtering. The SMA connectors used on this board are rated for 165°C, so they must be removed when testing at elevated temperatures for extended periods of time. Likewise, the insulation on the 0.1” header connectors (J2 and P3) only lasts a short time at high temperatures and must be removed during long-duration high-temperature testing.

PCB layout and assembly

In the PCB design of this circuit, the analog signal and digital interface are on opposite sides of the ADC, with no switching signals under the IC or near the analog signal path. This design minimizes noise coupling into the ADC chip and auxiliary analog signal chain. The AD7981 has all analog signals on the left and all digital signals on the right, a pinout that simplifies design. The reference voltage input REF has dynamic input impedance and must be decoupled with minimal parasitic inductance. To this end, the reference voltage decoupling capacitor must be placed as close as possible to the REF and GND pins, and a low-impedance wide trace should be used to connect the pins. foot. The components of this circuit board are all intentionally placed on the front to facilitate temperature testing by heating from the back. For additional 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 commonly used material for PCB stack-ups, but the typical glass transition temperature of commercial FR4 is about 140°C. Above 140°C, PCBs begin to crack, delaminate, and stress components. A widely used alternative material for high-temperature assembly is polyimide, which has a typical glass transition temperature greater than 240°C. This design uses a 4-layer polyimide PCB.

The PCB surface also requires attention, especially when used with solder containing tin, because this solder is prone to forming intermetallic compounds with copper traces. Nickel-gold surface treatment is often used, where nickel provides a barrier and gold provides a good surface for joint welding. In addition, high melting point solder must be used with an appropriate margin between the melting point and the maximum operating temperature of the system. SAC305 lead-free solder was selected for this assembly, with a melting point of 217°C, giving a 42°C margin over the maximum operating temperature of 175°C.

Performance expectations

The AD7981 has a typical SNR rating of 91 dB using a 1 kHz input tone and a 5 V reference voltage. However, when using a lower reference filter (as is often the case with low-power/low-voltage systems), SNR performance will degrade. According to the performance curves in the AD7981 data sheet, the expected SNR is approximately 86 dB at room temperature and a 2.5 V reference voltage. This SNR value is in good agreement with the performance achieved when testing this circuit at room temperature (approximately 86 dB SNR), as shown in Figure 8.

As the temperature increases to 175°C, the SNR performance only decreases to approximately 84 dB, as shown in Figure 9. THD is still better than −100 dB, as shown in Figure 10. A summary of the FFT for this circuit at 175°C is shown in Figure 11.

Figure 9. SNR versus temperature (1 kHz input tone, 580 kSPS)

Figure 10. THD versus temperature (1 kHz input tone, 580 kSPS)

Figure 11. AC performance at 1 kHz input tone, 580 kSPS, 175°C