SECTION 6 MULTICHANNEL APPLICATIONS Data Acquisition System Considerations Multiplexing Filtering Considerations for Data Acquisition Systems SHA and ADC Settling Time Requirements in Multiplexed Applications Complete Data Acquisition Systems on a Chip Multiplexing into Sigma-Delta ADCs Simultaneous Sampling Systems Data Distribution Systems using Multiple DACS
1 SECTION 6 MULTICHANNEL APPLICATIONS Data Acquisition System Considerations Multiplexing Filtering Considerations for Data Acquisition Systems SHA and ADC Settling Time Requirements in Multiplexed Applications Complete Data Acquisition Systems on a Chip Multiplexing into Sigma-Delta ADCs Simultaneous Sampling Systems Data Distribution Systems using Multiple DACs
SECTION 6 MULTICHANNEL APPLICATIONS Walt Kester. Wes Freeman DATA ACQUISITION SYSTEM CONFIGURATIONS There are many applications for data acquisition systems in measurement and process control. All data acquisition applications involve digitizing analog signals for analysis using ADCs. In a measurement application, the ADC is followed by a digital processor which performs the required data analysis. In a process control application, the process controller generates feedback signals which typically must be converted back into analog form using a dac Although a single adc digitizing a single channel of analog data constitutes a data acquisition system, the term data acquisition generally refers to multi-channel systems. If there is feedback from the digital processor, DACs may be required to convert the digital responses into analog. This process is often referred to as data distribution Figure 6.1 shows a data acquisition/distribution process control system where ead channel has its own dedicated ADC and DAC. an alternative configuration is shown in Figure 6.2, where analog multiplexers and demultiplexers are used with a single ADC and DAC. In most cases, especially where there are many channels, this configuration provides an economical alternative DATA ACQUISITION SYSTEM USING ADC/ DAC PER CHANNEL PROCESS CONTROLLER +2 DAC ADC PROCESS DAC Figure 6.1
2 SECTION 6 MULTICHANNEL APPLICATIONS Walt Kester, Wes Freeman DATA ACQUISITION SYSTEM CONFIGURATIONS There are many applications for data acquisition systems in measurement and process control. All data acquisition applications involve digitizing analog signals for analysis using ADCs. In a measurement application, the ADC is followed by a digital processor which performs the required data analysis. In a process control application, the process controller generates feedback signals which typically must be converted back into analog form using a DAC. Although a single ADC digitizing a single channel of analog data constitutes a data acquisition system, the term data acquisition generally refers to multi-channel systems. If there is feedback from the digital processor, DACs may be required to convert the digital responses into analog. This process is often referred to as data distribution. Figure 6.1 shows a data acquisition/distribution process control system where each channel has its own dedicated ADC and DAC. An alternative configuration is shown in Figure 6.2, where analog multiplexers and demultiplexers are used with a single ADC and DAC. In most cases, especially where there are many channels, this configuration provides an economical alternative. DATA ACQUISITION SYSTEM USING ADC / DAC PER CHANNEL Figure 6.1
DATA ACQUASITION SYSTEM USING ANALOG MULTIPLEXING/DEMULTIPLEXING AND SINGLE ADC/ DAC DAC CONTROLLER ADC ANALOG PROCESS ANALOG Figure 6.2 There are many tradeoffs involved in designing a data acquisition system Issues such as filtering, amplification, multiplexing, demultiplexing, sampling frequency and partitioning must be resolved. MULTIPLEXING Multiplexing is a fundamental part of a data acquisition system. Multiplexers and switches are examined in more detail in Reference 1. but a fundamental understanding is required to design a data acquisition system. a simplified diagram of an analog multiplexer is shown in Figure 6.3. The number of input channels typically ranges from 4 to 16, and the devices are generally fabricated on CMOs processes. Some multiplexers have internal channel-address decoding logic and registers, while with others, these functions must be performed externally. Unused multiplexer inputs must be grounded or severe loss of system accuracy may result The key specifications are switching time, on-resistance, on-resistance modulation and off-channel isolation(crosstalk Multiplexer switching time ranges from about 50ns to over lus, on-resistance from 25ohms to several hundred ohms, and off channel isolation from 50 to 90dB. The use of trench isolation has eliminated latch up in multiplexers while yielding improvements in speed at low supply voltages
3 DATA ACQUASITION SYSTEM USING ANALOG MULTIPLEXING / DEMULTIPLEXING AND SINGLE ADC / DAC Figure 6.2 There are many tradeoffs involved in designing a data acquisition system. Issues such as filtering, amplification, multiplexing, demultiplexing, sampling frequency, and partitioning must be resolved. MULTIPLEXING Multiplexing is a fundamental part of a data acquisition system. Multiplexers and switches are examined in more detail in Reference 1, but a fundamental understanding is required to design a data acquisition system. A simplified diagram of an analog multiplexer is shown in Figure 6.3. The number of input channels typically ranges from 4 to 16, and the devices are generally fabricated on CMOS processes. Some multiplexers have internal channel-address decoding logic and registers, while with others, these functions must be performed externally. Unused multiplexer inputs must be grounded or severe loss of system accuracy may result. The key specifications are switching time, on-resistance, on-resistance modulation, and off-channel isolation (crosstalk). Multiplexer switching time ranges from about 50ns to over 1µs, on-resistance from 25ohms to several hundred ohms, and offchannel isolation from 50 to 90dB. The use of trench isolation has eliminated latchup in multiplexers while yielding improvements in speed at low supply voltages
SIMPLIFIED DIAGRAM OF A TYPICAL ANALOG MULTIPLEXER CH ADDRESS CLOCK DECODER BUFFER, SHA, PGA, ADC ANALOG RL MUX Figure 6.3 MULTIPLEXER KEY SPECIFICATIONS Switching Time: 50ns to >lus On-Resistance: 25o to hundreds of os On-Resistance Modulation(Ron change with signal level Off-Channel isolation 50 to 90 dB Overvoltage Protection Figure 6.4
4 SIMPLIFIED DIAGRAM OF A TYPICAL ANALOG MULTIPLEXER Figure 6.3 MULTIPLEXER KEY SPECIFICATIONS Switching Time: 50ns to >1 s On-Resistance: 25 to hundreds of ’s On-Resistance Modulation (Ron change with signal level) Off-Channel Isolation: 50 to 90 dB Overvoltage Protection Figure 6.4
WHAT'S NEW IN MULTIPLEXERS? Trench Isolation gives high speed, latch-up protection, and low voltage operation ADG511, ADG512, ADG513: +3. 3v, +5V,#5V specified Ron<509@±5V Switching Time: <200ns@+5V ADG411, ADG412, ADG413:+15v, +12V specified Ron<3592@ ±15 V Switching Time:<150ns@±15V ADG508F, ADG509F, ADG528F:+15V specified Ron 300Q Switching Time:≤250ns Fault-Protection on Inputs and Outputs Figure 6.5 Multiplexer on-resistance is generally slightly dependent on the signal level(often called Ron modulation). This will cause signal distortion if the multiplexer must drive a load resistance, therefore the multiplexer output should therefore be isolated from the load with a suitable buffer amplifier. a separate buffer is not required if the multiplexer drives a high input impedance, such as a Pga, sha or ADC-but beware! Some SHAs and ADCs draw high frequency pulse current at their sampling rate and cannot tolerate being driven by an unbuffered multiplexer. a detailed analysis of multiplexers can be found in Reference 1, Section 8, or Reference 2, Section 2 An M-channel multiplexed data acquisition system is shown in Figure 6.6. The multiplexer output drives a Pga whose gain can be adjusted on a per- channel basis depending on the channel signal level. This ensures that all channels utilize the full dynamic range of the ADC. The PGa gain is changed at the same time as the multiplexer is switched to a new channel. The AdC Convert Command is applied fter the multiplexer and the pga have settled to the required accuracy(ILSB). The maximum sampling frequency(when switching between channels)is limited by the multiplexer switching time tmux, the Pga settling time tmga, and the adc conversion time tony as shown in the formula
5 WHAT’S NEW IN MULTIPLEXERS? Trench Isolation gives high speed, latch-up protection, and lowvoltage operation ADG511, ADG512, ADG513: +3.3V, +5V, 5V specified Ron < 50 @ 5V Switching Time: <200ns @ 5V ADG411, ADG412, ADG413: 15V, +12V specified Ron < 35 @ 15V Switching Time: <150ns @ 15V ADG508F, ADG509F, ADG528F: 15V specified Ron < 300 Switching Time: < 250ns Fault-Protection on Inputs and Outputs Figure 6.5 Multiplexer on-resistance is generally slightly dependent on the signal level (often called Ron modulation). This will cause signal distortion if the multiplexer must drive a load resistance, therefore the multiplexer output should therefore be isolated from the load with a suitable buffer amplifier. A separate buffer is not required if the multiplexer drives a high input impedance, such as a PGA, SHA or ADC - but beware! Some SHAs and ADCs draw high frequency pulse current at their sampling rate and cannot tolerate being driven by an unbuffered multiplexer. A detailed analysis of multiplexers can be found in Reference 1, Section 8, or Reference 2, Section 2. An M-channel multiplexed data acquisition system is shown in Figure 6.6. The multiplexer output drives a PGA whose gain can be adjusted on a per-channel basis depending on the channel signal level. This ensures that all channels utilize the full dynamic range of the ADC. The PGA gain is changed at the same time as the multiplexer is switched to a new channel. The ADC Convert Command is applied after the multiplexer and the PGA have settled to the required accuracy (1LSB). The maximum sampling frequency (when switching between channels) is limited by the multiplexer switching time tmux, the PGA settling time tpga, and the ADC conversion time tconv as shown in the formula