Monitoring & metering
from the mixture to two individual samples is shown in Figure 2. Green and purple colours stand for two different samples, and blue colour stands for mixture.
GAS CHROMATOGRAPHY SYSTEM SOLUTION Based on Figure 1, Analog Devices has various components that can be applicable to the signal and power supply chain of gas chromatography. These components are well-suited to meet system requirements and are optimised for low power and noise performance.
Air Inlet System
Great precision is required to control the flow of the carrier gas accurately passing through the detector system to separate the mixture, which is realised by the use of a gas sensor. A gas sensor has diverse outputs such as a current signal, 4–20mA loop signal, voltage signal, etc. A signal chain of an air inlet system is depicted in Figure 3.
Current signal: The ADA4530-1 is a femtoampere level input bias current operational amplifier used as the I/V (current to voltage) conversion, and it provides ultralow input bias currents (femtoampere) that ensure the device meets accurate performance goals in the system. It also offers low offset voltage, low offset drift, and low voltage and current noise needed for the types of applications that require such low leakages.
After converting the current signal into a voltage signal, the AD7175 (24-bit, 8-/16- channel, 250 kSPS, sigma-delta ADC) is used to get accurate results. The AD7175 is a low noise, fast settling, multiplexed, 8-/16-channel (fully/pseudo differential) sigma-delta ADC for low bandwidth inputs.
4–20mA signal sampling: the AD4111 ADC is a good choice for sampling the air sensor outputs transmitted by 4– 20mA signal, as it has extremely good channel matching due to the integrated sense resistors for the current and voltage sampling. The device is a low power, low noise, 24-bit, sigma-delta ADC that integrates an analogue front end (AFE) for fully differential or single- ended, high impedance (≥1 M ) bipolar, ±10 V voltage inputs, and 0 mA to 20 mA current inputs. This ADC also integrates key analog and digital signal conditioning blocks to configure eight individual setups for each analogue input channel in use. It features a maximum channel scan rate of 6.21 kSPS (161 µs) for fully settled data and it also has the unique feature of open wire detection on the voltage inputs for system--level diagnostics using a single 5 V or 3.3 V power supply.
Instrumentation Monthly August 2024 Figure 3. The signal chain of an air inlet system.
Figure 4. An ADA4530-1 data sheet.
Voltage signal: If an external ADC with high resolution is selected, the ADR4xxx series (that is, ADR4525/ADR4530) can provide a high accuracy reference voltage for the ADC. Alternatively, a low cost way is to use the internal ADC of the microcontroller unit (MCU) to perform the sampling, but this may sacrifice the measurement accuracy.
Temperature Control System The temperature is controlled by a closed-loop system: The LT1241 is used to drive a MOSFET bridge with pulse width modulation (PWM) as thermoelectric cooler (TEC) circuitry to control the thermal cooler, and a resistance temperature detector (RTD) reads back the temperature and connects to the AD7124, an ultralow noise and
power sigma-delta ADC. The AD7124 is intended for temperature measurement applications. To improve temperature acquisition accuracy, it can support a 2-wire thermocouple and is optimised for a 3-wire RTD configuration catering to different customers’ expectations. The ADN8835 that integrates the TEC controller is also available. The temperature control system diagram is shown in Figure 5.
Detector System
The detector system is the final stage, which is used for describing the chromatogram. The flow and pressure sensors are used for monitoring the carrier gas flowing movement with different sampling requests, so the AD7124 and LTC2498 with multichannels are selected. The ion current signal is normally picoampere level entering the detector and the ADA4530-1
Figure 5. A signal chain of a temperature control system.
Continued on page 64... 63
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100
