1 Overview

Ads1274 / ADS1278 is a multi-channel 24 bit industrial analog-to-digital converter (ADC) launched by Texas Instruments (TI). It integrates multiple independent high-order chopper stable modulators and FIR digital filters. It can realize 4 / 8-channel synchronous sampling and support four working modes of high speed, high precision, low power consumption and low speed; Ads1274 / ADS1278 has excellent AC and DC characteristics, the highest sampling rate can reach 128 KS / s, the signal-to-noise ratio (SNR) can reach 111 dB at 62 kHz bandwidth, and the offset drift is 0.8 μ V/℃。

Ads1274 / ADS1278 can select the working mode by setting the corresponding input / output pins without register programming. Its data output can choose frame synchronization or SPI serial interface, which is easy to connect to DSP, FPGA and microcontroller. Each interface supports daisy chain to simplify the readback function of multiple ads1274 or ADS1278 in the multi-channel counting system. The working temperature range of ads1274 is – 40 ℃ ~ + 125 ℃, while that of ADS1278 is – 40 ℃ ~ + 105 ℃, which can meet the strict requirements of multi-channel signal acquisition applications, including vibration analysis, medical monitoring, acoustic / dynamic strain measurement and pressure measurement equipment.

2 performance characteristics

The traditional industrial △ – ∑ ADC with high drift performance uses a digital filter with large conduction bandwidth slip amplitude to meet the limited signal bandwidth of DC measurement as much as possible. High resolution ADCs for audio applications can provide greater available bandwidth, but the offset and drift specifications are much lower than industrial ADCs. Ads1274 / ADS1278 combines two types of converters to achieve high-precision industrial measurement of the best DC and AC specifications. The specific features are as follows:

Allow 4 / 8 channels of synchronous sampling;

The sampling rate is up to 128 KS / S;

AC performance: when working in high-precision mode and the input signal bandwidth is 62 kHz, the SNR is 111 dB and the total harmonic distortion (THD) is – 108 db;

DC performance: 0.8 μ V / ℃ offset drift, 1.3 ppm / ℃ gain drift;

Four optional working modes: high speed mode is 128 KS / s, signal-to-noise ratio is 106 db; The high-precision mode is 52 KS / s and the signal-to-noise ratio is 111 db; The low power consumption mode is 52 KS / s, 31 MW / channel; The low speed mode is 10 KS / s, 7 MW / channel;

On chip linear phase digital filter;

SPI or frame synchronous serial interface;

Low sampling aperture error;

Modulator output selection.

3 pin function description

Ads1274 / 1278 are packaged in 64 pin htqfp. Their pins are compatible for easy expansion. The pin function description is shown in Table 1.

Performance characteristics and application analysis of multichannel 24 bit industrial analog-to-digital converter

4 function description

4.1 working principle

Ads1274 / ADS1278 are △ – ∑ analog-to-digital converters. Their structural block diagram is shown in Figure 1. They have the same functions except for the number of internal analog-to-digital converters (ADCs). They are mainly composed of 4 / 8 independent ADCs to realize the digitization of 4 / 8 channel input signals in parallel. Each ADC is composed of advanced 6th order chopper, △ – ∑ modulator, followed by low ripple, linear phase finite impulse response (FIR) digital filter. The modulator detects the differential input signal VIN = (ainp-ainn) and compares it with the differential reference voltage Vref = (vrefp-vrefn) to obtain a 1-second density bit stream output. The output bits are filtered by an internal digital filter to obtain a low-noise digital output.

During operation, the modulator samples the input signal at high speed (64 times of the typical output data rate), and the quantization noise generated by the modulator is moved to the high frequency band and filtered by its internal digital filter. The oversampling magnification of the modulator is related to the working mode, which can be 64 (high-speed, low-speed and low-power mode) or 128 (high-precision mode) respectively, The digital filter can attenuate the signal beyond the cut-off frequency by 100 dB, so that the ripple is less than 0.005 dB when the signal conduction bandwidth is 90% of the Nyquist frequency.

4.2 working mode setting

Ads1274 / ADS1278 allow the trade-off in speed, accuracy and power consumption. Select one of the four working modes. The performance under various working modes is shown in Table 2, and the mode selection is determined by the input state of mode [1:0] pin.

4.3 data output interface

The data output after ads1274 / ADS1278 conversion adopts serial interface. Two interface protocols can be adopted: SPI protocol and frame synchronization protocol. At the same time, different data output formats can be selected. The selection of protocol and data output format is determined by the input state of forfmat [2:0] pin.

Corresponding to SPI and frame synchronization interface protocol, the converted data is shifted and output in the form of parallel data (discrete mode) through an independent dout pin or shifted and output through a common pin dout1 (TDM mode).

In TDM data output mode, there are two options for the location of multi-channel output data, namely, fixed location and dynamic location allocation. When the position is fixed, the output data of each channel is output in strict order (even if some channels are powered down, they also occupy an output position). During dynamic allocation, the position of multi-channel output data can be adjusted randomly according to the channel usage, and the subsequent channel data can occupy the position where the front channel data is not used in power down. Fig. 2 and Fig. 3 describe the TDM mode, Table 3 lists the interface protocol, output format and data location relationship.

5 application interface circuit

5.1 basic differential input signal interface circuit

The basic differential input signal interface circuit is shown in Figure 4. The low-pass filter composed of operational amplifier opa1632 and RC is used to condition the input signal. The VCOM end can be directly connected to the VCOM pin of ADC or buffered output through opa350. The 12 V power supply can be 10 v μ F and 0.1 μ F capacitor bypass. For capacitor (2), a capacitor of 2.7 NF can be selected when working in low-power mode, and a capacitor of 15 NF can be selected when working in low-speed mode.

5.2 interface circuit between ads1724 and TMS320

Figure 5 shows the interface circuit between asd1274 and TMS320VC5509. Frame synchronous serial interface is adopted between ads1274 and TMS320VC5509, clkdiv is connected to 3.3 V, the sampling master clock frequency is 27 MHz, and four analog input signals can be connected to ainp and ainn by the connection shown in Figure 4.

6 Conclusion

Ads1274 / 1278 is a 24 bit high-performance industrial analog-to-digital converter based on △ – ∑ technology. It integrates multiple independent high-order chopper stable modulators and FIR digital filters. It has excellent AC and DC performance, can realize 4 / 8-channel synchronous sampling, and supports four working modes of high speed, high precision, low power consumption and low speed; Frame synchronization or SPI serial interface can be selected for data output. Each interface supports daisy chain connection and can be applied to multi-channel signal acquisition systems with strict requirements, such as vibration analysis, medical monitoring, dynamic strain measurement equipment, etc.

Responsible editor: GT

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