Complete High Speed, High CMRR Precision Analog Front End for Process Control
The analog front-end circuit shown in Figure 1 is optimized for high precision and high common-mode rejection ratio (CMRR) when processing these types of industrial-level signals.
Figure 1: High Performance Analog Front End for Process Control (Simplified Schematic: All Connections and Decoupling Not Shown)
The circuit level shifts and attenuates the signals so they are compatible with the input range requirements of most modern single-supply SAR ADCs, such as the AD7685 high performance 16-bit 250 kSPS PulSAR® ADC.
With an 18 V p-p input signal, the circuit achieves approximately 105 dB common-mode rejection (CMR) at 100 Hz and 80 dB CMR at 5 kHz.
High precision, high input impedance, and high CMR are provided by the AD8226 instrumentation amplifier. For high precision applications, a high input impedance is required to minimize system gain errors and also to achieve good CMR. The AD8226 gain is resistor-programmable from 1 to 1000.
A resistive level shifter/attenuator stage directly on the input would inevitably degrade CMR performance due to the mismatch between the resistors. The AD8226 provides the excellent CMR required for both small signal and large signal inputs. The AD8275 level shifter/attenuator/driver performs the attenuation and level shifting function in the circuit, without any need for external components.
Traditionally, sigma-delta ADCs have been used in high resolution measurement systems because signal bandwidths are quite low, and the sigma-delta architecture provides excellent noise performance at low update rates. However, there is an increased trend for higher update rates, especially in multichannel systems, to allow faster per-channel update, or for increased channel density. In such cases a high performance SAR ADC is a good alternative. The circuit shown in Figure 1 uses the AD7685 250 kSPS 16 bit ADC, with the AD8226 high performance in-amp, and the AD8275 attenuator/level shifter amplifier implemented as a complete system solution without the need for any external components.
The ADP1720 is used to provide the 5 V supply for the AD8275 and the AD7685. The ADP1720 was chosen because it has a high input voltage range (up to 28 V). In this circuit, the ADP1720 is only required to supply approximately 4 mA to the AD8275 and AD7685, so the worst case power dissipation in the regulator with 28 V input is about 90 mW. This allows the complete system to run from the external ±15 V supplies.
System Level Common-Mode Rejection Performance
Initial testing was to verify the performance of the AD8226 common-mode rejection at a system level through to the ADC. Input test tones of 10 Hz, 100 Hz, 500 Hz, 1 kHz, 2 kHz, 3 kHz, 4 kHz, 5 kHz were used, with an 18 V p-p input signal. The test results are summarized in Table 1. In Test 1 the AIN+ and AIN− signals are shorted together and connected to the ac test tone, and the results measured with an FFT. The AD8226 should reject the ac signal because its inputs are connected together. In Test 2 the signal is applied to AIN+, and AIN− is connected to ground. Under these conditions, the FFT measures the level of the tone. The common-mode rejection can then be calculated as the difference in the FFT results between Test 1 and Test 2. Table 1 summarizes the CMR values obtained at different frequencies. It is important to note that the CMR of the AD8226 is specified at 80 dB at 5 kHz, so no loss in CMR performance is realized at the system level.
System Level AC Performance
The ac accuracy of the system was also tested at a system level with the AD7685 operating at a sampling rate of 250 kSPS. Figure 2 shows an FFT test result for a 5 V p-p input at 10 kHz. The results shown in the plot are given below:
- SNR = 87.13 dBFS
- SINAD = 85.95 dBFS
- SFDR = 81.82 dBc
- THD = −78.02 dBc
Table 1: CMR Performance of Circuit for 18 V p-p Input
Figure 2: Figure 2. FFT Results for 10 kHz Input Signal 14 dB Below Full-Scale, 250 kSPS
The performance of this or any high speed circuit is highly dependent on proper PCB layout. This includes, but is not limited to, power supply bypassing, controlled impedance lines (where required), component placement, signal routing, and power and ground planes. (See MT-031 Tutorial, MT-101 Tutorial, and article, A Practical Guide to High-Speed Printed-Circuit-Board Layout, for more detailed information regarding PCB layout.)
A complete design support package for this circuit note can be found at http://www.analog.com/CN0213-DesignSupport.
Equipment Used to Collect Test Data
- PC with a USB port and Windows® XP or Windows Vista® (32-bit), or Windows® 7 (32-bit)
- EVAL-A-INPUT-1AZ circuit evaluation board
- EVAL-SDP-CB1Z, SDP-A evaluation board
- Evaluation software
- Power supply: +5 V @ 200 mA
- Power supply: ±15 V, Agilent E3630A or equivalent
- Signal generator: Agilent 33120A or equivalent
Figure 4: Photo of EVAL-A-INPUT-1AZ Evaluation Board Connected to SDP Board
Setup and Test
Evaluation software was loaded in the CD drive of the PC.
The 120-pin connector on the EVAL-A-INPUT-1AZ circuit board was connected to the connector marked “CON B” on the EVAL-SDP-CB1Z evaluation (SDP) board. Nylon hardware was used to firmly secure the two boards, using the holes provided at the ends of the 120-pin connectors. The signal source was connected to the EVAL-A-INPUT-1AZ board J1 input (AIN+) terminal. When running normal FFT tests, the JP1 jumper was connected between the J3 terminal (IN−) and ground. When running CMR tests, the jumper was connected between J1 (AIN+) and J3 (AIN−).
With power to the supply off, a +5 V power supply was connected to the SDP board. The USB cable was used to connect the SDP board to the USB port on the PC.
The ±15 V supplies were then connected to EVAL-A-INPUT-1AZ circuit board. The evaluation software was launched, and the USB cable was connected from the PC to the USB mini-connector on the SDP board.
Once USB communications are established, the SDP board was used to send, receive, and capture serial data from the EVAL-A-INPUT-1AZ board.
|AD7685||16-Bit, 250 kSPS PulSAR® ADC in MSOP/QFN||
|AD8226||Wide Supply Range, Rail-to-Rail Output Instrumentation Amplifier||
|AD8275||G = 0.2, Level Translation, 16-Bit ADC Driver||
|ADP1720||50 mA, High Voltage, Micropower Linear Regulator||
|ADR439||Ultralow Noise XFET® 4.5V Voltage Reference w/Current Sink and Source Capability|