AN-2529: Complete High Speed, High CMRR Precision Analog Front End for Process Control

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Circuit Function and Benefits

Signal levels in industrial process control systems generally fall into one of the following categories: single-ended current (4 mA-to-20 mA), single-ended, differential voltage (0 V to 5V, 0 V to 10 V, ±5 V, ±10 V), or small signal inputs from sensors such as thermocouples or load cells. Large common-mode voltage swings are also typical, especially for small signal differential inputs; therefore good common-mode rejection is an important specification in the analog signal processing system.

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).

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.

Circuit Description

This circuit is comprised of an AD8226 rail-to-rail output in amp, connected to the positive input of the AD8275 differential amplifier, whose output is then connected to the input of the AD7685, a 16- bit, 250 kSPS PulSAR ADC. The AD8226 is set for a gain-of-one mode (high voltage/current inputs), and its output is referenced to ground. Single-ended or differential inputs can be used, and the output of the AD8226 is a bipolar signal, which drives the AD8275 input.

The AD8275 attenuates and level shifts the bipolar input, providing a gain of 0.2. Hence, differential inputs of 20 V p-p yield a 4 V p-p single-ended range on the output. The ADR439 precision 4.5 V reference provides both the internal common-mode bias voltage for the AD8275 (VREF/2 = 2.25 V), as well as the external reference voltage for the AD7685 ADC. Under these conditions, the output of the AD8275 swings from +0.25 V to +4.25 V, which is within the range of the AD7685 (0 V to +4.5 V).

The ADP1720 low dropout regulator provides 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 using a ±15 V external supply.

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
FFT Signal Level (dBFS), 18 V p-p Input 
Frequency (kHz) Test 1: AIN+ = AIN– (dBFS) Test 2: Input = AIN+, AIN– = GND (dBFS) Test 2: Input = AIN+, AIN– = GND (dBFS)
0.1 −104.64 −2.86  101.78 
0.5 −100.00  −3.28  96.72
1 −94.67 −2.85 91.82
2 −88.58  −2.88 85.70
3 −84.93 −2.93 82.00
4 −82.07 −3.01 79.06
5 −79.43  −3.10  76.33

Figure 2. FFT Results for 10 kHz Input Signal 14 dB Below Full-Scale, 250 kSPS Sampling Rate.

Figure 2. FFT Results for 10 kHz Input Signal 14 dB Below Full-Scale, 250 kSPS Sampling Rate.

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.)