24-Bit, 4.7 Hz, 4-Channel Analog Data Acquisiton System
The circuit shown in Figure 1 is a flexible signal conditioning circuit for processing signals of wide dynamic range, varying from several mV p-p to 20 V p-p. The circuit provides the necessary conditioning and level shifting and achieves the dynamic range using the internal programmable gain amplifier (PGA) of the high resolution analog-to-digital converter (ADC).
A ±10 V full-scale signal is very typical in process control and industrial automation applications; however, in some situations, the signal can be as small as several mV. Attenuation and level shifting is necessary to process a ±10 V signal with modern low voltage ADCs. However, amplification is needed for small signals to make use of the dynamic range of the ADC. Therefore, a circuit with a programmable gain function is desirable when the input signal varies over a wide range.
In addition, small signals may have large common-mode voltage swings; therefore, high common-mode rejection (CMR) is required. In some applications, where the source impedance is large, high impedance is also necessary for the analog front-end input circuit.
Figure 1. Flexible Analog Front-End Circuit for Wide Industrial Range Signal Conditioning
The circuit shown in Figure 1 solves all of these challenges and provides programmable gain, high CMR, and high input impedance. The input signal passes through the 4-channel ADG1409 multiplexer into the AD8226 low cost, wide input range instrumentation amplifier. The AD8226 offers high CMR up to 80 dB and very high input impedance (800 MΩ differential mode and 400 MΩ common mode). A wide input range and rail-to-rail output allow the AD8226 to make full use of the supply rails.
The AD8475 is a fully differential, attenuating amplifier with integrated precision gain resistors. It provides precision attenuation (G = 0.4 or G = 0.8), common-mode level shifting, and single-ended-to-differential conversion. The AD8475 is an easy to use, fully integrated precision gain block, designed to process signal levels up to ±10 V on a single supply. Therefore, the AD8475 is suitable for attenuating signals from the AD8226 up to 20 V p-p, while maintaining high CMR and offering a differential output to drive the differential input ADC.
The AD7192 is a 24-bit sigma-delta (Σ-Δ) ADC with an internal PGA. The on-chip, low noise gain stage (G = 1, 8, 16, 32, 64, or 128) means that signals of large and small amplitude can be interfaced directly to the ADC.
With the combination of the previous parts, the circuit offers very good performance and easy configuration for signals with varying amplitudes. The circuit can be used in industrial automation, process control, instrumentation, and medical equipment applications.
The circuit comprises of an ADG1409 multiplexer, an AD8226 instrumentation amplifier, an AD8475 difference amplifier, and an AD7192 Σ-Δ ADC with an ADR444 reference, and the ADP1720 regulator. Only a few external components are used for protection, filtering, and decoupling, making this circuit highly integrated, and it requires a small circuit board (printed circuit board [PCB]) area.
Regulator and Reference Selection
The ADP1720-5 was chosen as the 5 V regulator for this circuit. It is a high voltage micropower, low dropout linear regulator suitable for industrial applications.
The 4.096 V ADR444 reference was chosen as the reference for this circuit. It is an ultralow noise, high accuracy, low dropout device that is particularly suitable for high resolution, Σ-Δ ADCs and precision data acquisition systems.
Input Switch and Protection
The ADG1409 multiplexer has 2-bit binary address lines that are used to select one of four possible input channels. The design also includes external protection such as standard diodes and transient voltage suppressors to enhance the robustness of the circuit. These are not shown in Figure 1; however, they are shown in the detailed schematics and other documentation in the CN0251 Design Support Package.
The ADG1409 multiplexer is configured to accept four differential input signals: (VS1A − VS1B), (VS2A − VS2B), (VS3A − VS3B), and (VS4A − VS4B). The outputs of the multiplexer, DA and DB, are applied to the inputs of the AD8226 in-amp.
AD8226 Input Instrumentation Amplifier
The external RG resistor sets the gain of the AD8226. For this circuit, RG is omitted, and the gain of the in-amp stage is 1. The output of the AD8226 is therefore VSxA - VSxB, where x is the input channel number.
The differential input of the AD8226 is filtered by two 4.02 kΩ resistors and a 10 nF capacitor, which form a single-pole RC filter with a cutoff frequency of 2.0 kHz. The two 1 nF capacitors add common-mode filtering with a cutoff frequency of 40 kHz.
AD7192 ADC PGA Gain Configuration
The AD7192 is configured to accept differential analog inputs to match the differential output signals from the AD8475. The full-scale input range of the AD7192 is ±VREF/gain, where ±VREF = REFINx(+) - REFINx(-).
When the buffer in the AD7192 is enabled, the input channel drives the high impedance input stage of the buffer amplifier, and the absolute input voltage range in this mode is restricted to a range of AGND + 250 mV and AVDD - 250 mV. When the gain stage is enabled, the output from the buffer is applied to the input of the PGA, and the analog input range must be limited to ±(AVDD - 1.25 V)/gain because the PGA requires additional headroom. Therefore, with a 4.096 V reference and a 5 V power supply, and to make the maximum use of the dynamic range of the ADC, the signal can be attenuated or amplified as shown in Table 1.
Table 1. Gain Configurations for the AD8475 and the AD7192 Internal PGA for Various Input Ranges
|Input Range (VSxA − VSxB)||Gain of AD8475||Gain of AD7192||PGA Output Range, Bipolar Mode (V)|
|± 125 mV||0.8||32||±3.2|
Differential Attenuating Amplifier
To drive the low voltage ADCs, the ±10 V or ±5 V signals require attenuation and level shifting. A difference amplifier configuration in conjunction with precision resistors inevitably degrades CMR performance due to the mismatch between the resistors. The AD8475 level shifter/attenuator integrates matched, precision, laser trimmed resistors to ensure low gain error, a low gain drift (3 ppm/°C maximum), and high CMR.
The AD8475 has pin-selectable gain options of 0.4 and 0.8. The VOCM pin adjusts the output voltage common mode for precision level shifting to match the input range of the ADC and to maximize the dynamic range. This pin can be left floating and is internally biased with a precision voltage divider consisting of two 200 MΩ resistors between the supplies and ground, thereby providing the midsupply voltage on the pin.
A single-pole differential RC filter composed of two 100 Ω resistors and a 1 μF capacitor serves as an antialiasing and noise reduction filter for the AD7192 with a cutoff frequency of 800 Hz. The two 10 nF capacitors provide common-mode filtering with a cutoff frequency of 160 kHz.
Filter, Output Data Rate, and Settling Time
The AD7192 Σ-Δ ADC consists of a modulator followed by a digital filter. The output data rate (fADC) and settling time (tSETTLE) are related to the filter configuration and the chop configuration. Table 2 shows the output data rates and settling time calculations for different configurations.
Table 2. Output Data Rate and Settling Time for Different Configurations
|Disabled|| fADC = fCLK/(1024 × FS[9:0])
tSETTLE = 3/fADC
|fADC = fCLK/(1024 × FS[9:0])
tSETTLE = 4/fADC
|Enabled|| fADC = fCLK/(3 × 1024 × FS[9:0])
tSETTLE = 2/fADC
| fADC = fCLK/(4 × 1024 × FS[9:0])
tSETTLE = 2/fADC
The performance of this or any other high speed or high resolution circuit is highly dependent on proper PCB layout. This includes, but is not limited to, power supply bypassing, signal routing, and proper power planes and ground planes. See Tutorial MT-031, Tutorial MT-101, and the article A Practical Guide to High-Speed Printed-Circuit-Board Layout for more detailed information regarding PCB layout.
With the configuration set to chop disabled, an output data rate of 4.7 Hz, a gain of 1, and a SINC4 filter, Figure 2 shows the noise performance, and Figure 3 shows the noise distribution histogram with 500 samples. The measured peak-to-peak noise of this circuit is approximately 3.9 µV (see Figure 2), and the rms noise is 860 nV. This corresponds to a peak-to-peak (noise-free code) resolution of 20 bits and an rms resolution of 23 bits. Table 3 shows the rms noise for some of the data rates and gain settings of the AD7192 with chop disabled and a SINC4 filter.
Figure 2. Noise Output (VREF = 4.096 V, AVDD = 5 V, Output Data Rate = 4.7 Hz, a Rate = 4.7 Hz,
Figure 3. Noise Histogram (VREF = 4.096 V, AVDD =5 V, Output Data Rate = 4.7Hz, Gain = 1, Chop Disabled, SINC4 Filter)
Table 3. RMS Resolution of the System with Different Output Data Rates and Gain Settings of the AD7192 with Chop Disabled and a SINC4 Filter (Subtract 2.7 Bits to Get Peak-to-Peak or Noise-Free Code Resolution)
|Filter Word (Decimal)||Output Data Rate (Hz)||Settling Time (ms)||Gain 1||Gain 8||Gain 16||Gain 32||Gain 64||Gain 128|
Other 24-bit or lower resolution Σ-Δ ADCs with integrated PGAs can be used, such as the AD7190, AD7193, AD7797, and AD7799. If no attenuation for the input signal is required, the AD8476 can be used for lower power than the AD8475.
In applications where there is no need for attenuation and high input impedance, the AD7192 can be connected directly to the sensor to avoid noise introduced by the analog front-end conditioning circuits. For instance, a load cell with a small full-scale output voltage does not require attenuation and can be connected directly to the AD7192 differential inputs (see CN-0102, CN-0107, CN-0108, CN-0118, CN-0119, and CN-0155)
The following equipment is needed:
- A PC with a USB port and Windows XP or Windows Vista (32-bit) or Windows 7 (32-bit)
- An EVAL-CN0251-SDPZ circuit evaluation board
- An EVAL- SDP-CB1Z SDP evaluation board
- A dc supply: +15 V, -15 V, and +6 V.
- CN-0251 evaluation software
Load the evaluation software by placing the CN-0251 evaluation software CD in the CD drive of the PC. Then, locate the drive that contains the evaluation software CD and open the Readme file. Follow the instructions contained in the Readme file for installing and using the evaluation software.
Functional Block Diagram
Figure 4 shows a functional block diagram of the test setup. The EVAL-CN0251-SDPZ-SCH.pdf file has the detailed schematics for the EVAL-CN0251-SDPZ. This file is contained in the CN-0251 Design Support Package: www.analog.com/CN0251-DesignSupport.
Figure 4. Test Setup Functional Block Diagram
Connect the 120-pin connector on the EVAL-CN0251-SDPZ to the CONA connector on the EVAL-SDP-CB1Z (SDP). Use nylon hardware to firmly secure the two boards, using the holes provided at the ends of the 120-pin connectors. After successfully setting the dc output supply to the +15 V, -15 V, and +6 V output, turn the power supply off.
With power to the supply off, connect a +15 V power supply to the +15VA pin of J3, a -15 V power supply to the -15VA pin of J3, and GND to the AGND pin of J3. Also, with power to the supply off, connect 6 V to J2. Turn on the power supply and then connect the USB cable with the SDP to the USB port on the PC. Do not connect the USB cable to the mini-USB connector on the SDP before turning on the dc power supply for the EVAL-CN0251-SDPZ.
After setting up the power supply and connecting to EVAL-CN0251-SDPZ, launch the evaluation software and connect the USB cable from the PC to the mini-USB connector on the SDP. The software is able to communicate to the SDP if the Analog Devices System Development Platform driver is listed in the Device Manager.
Once USB communications are established, the SDP can now be used to send, receive, and capture serial data from the EVAL-CN0251-SDPZ. Then, connect the signal source for measurements.
Information regarding the SDP can be found at www.analog.com/SDP.
|ADG1409||4 Ω Ron, 4 - /8 - Channel ±15 V/+12 V/±5 V iCMOS Multiplexer||
|AD7192||4.8 kHz Ultra-Low Noise 24-Bit Sigma-Delta ADC with PGA||
|AD8226||Wide Supply Range, Rail-to-Rail Output Instrumentation Amplifier||
|AD8475||Precision, Selectable Gain, Fully Differential Funnel Amp||
|ADP1720||50 mA, High Voltage, Micropower Linear Regulator||
|ADR444||Ultralow Noise, LDO XFET® 4.096V Voltage Reference w/Current Sink and Source||