Low Cost (8-Channel) Thermocouple Signal Conditioning With Analog Multiplexer and Analog Optocoupler.

In the current project will be created a signal conditioning for thermocouple equipped with an analog multiplexer, the total input for this circuit is 8-channel thermocouple input.

Some of the problems in a thermocouple signal conditioning is required strengthening gain and also giving a pretty good filter so the noise do not stay in the thermocouple which has a weak signal.

Some of the main components in this project are:
1. Multiplexer Analog Input For Thermocouple (ADG507A)
2. Thermocouple Amplifier (AD595)
3. Op-Amp (INA126)
4. Analog Optocoupler (HCNR201)



ADG507A



The ADG507A are CMOS monolithic analog multiplexers with 16 channels and dual 8 channels, respectively. The ADG507A switches one of eight differential inputs to a common differential output, depending on the state of three binary addresses and an enable input. Both devices have TTL and 5 V CMOS logic compatible digital inputs. The ADG507A are designed on an enhanced LC2MOS process, which gives an increased signal capability of VSS to VDD and enables operation over a wide range of supply voltages. The devices can operate comfortably anywhere in the 10.8 V to 16.5 V single or dual supply range. These multiplexers also feature high switching speeds.


AD595



The AD595 is a complete instrumentation amplifier and thermocouple cold junction compensator on a monolithic chip. It combines an ice point reference with a precalibrated amplifier to produce a high level (10 mV/°C) output directly from a thermocouple signal. Pin-strapping options allow it to be used as a linear amplifier-compensator or as a switched output setpoint controller using either fixed or remote setpoint control. It can be used to amplify its compensation voltage directly, thereby converting it to a stand-alone Celsius transducer with a low impedance voltage output.

You can see the AD595 output voltage as a representation of the temperature thermocuople complete in datasheet. The Appropriate thermocouple type for the AD595 is J and K-type thermocouple.


INA126



The INA126 is precision instrumentation amplifiers for accurate, low noise differential signal acquisition. Their two-op-amp design provides excellent performance with very low quiescent current (175μA/channel). This, combined with a wide operating voltage range of ±1.35V to ±18V, makes them ideal for portable instrumentation and data acquisition systems. Gain can be set from 5V/V to 10000V/V with a single external resistor. Laser trimmed input circuitry provides low offset voltage (250μV max), low offset voltage drift (3μV/°C max) and excellent common-mode rejection.

You can see the basic connection and how to setting the gain for INA126 in the picture below.




How The Circuit Work

The complete picture thermocouple signal conditioning circuit as shown below.



Instal the thermocouple sensor in the channel that you want on the input terminals. For example if you want to put on channel 1 you can plug thermocouple in the terminal J1. Do addressing for ADG507A multiplexer in accordance with the input channel that you want. For reference ADG507A addressing you can use the reference to the ADG507A datasheet which you can download at the bottom part.

Thermocouple output on this ADG507A will be given to the AD595 as an information signal from the thermocouple. ADG595 will process this data and then converted and amplified into a voltage that represents the signal from the thermocouple. The magnitude of this voltage can you see on the AD595 datasheet.

The output of the AD595 is still too small so need to be strengthened again using the INA126 op-amp, Set INA126 Gain by adjusting the value of VR1 to get the gain that you want. Do not forget to set the offset of the INA126 with setting VR2, adjust VR2 until the voltage at pin5 of ICINA126 = +/- 0 Volt.


HCNR201



Actually the output of the INA126 is good enough to be used, but sometimes there are problems when the tip of this thermocouple we attach to the body of the equipment that containing the ground, therefore in this series are equipped with analog isolatar HCNR201 to isolate the output voltage of the INA126 to not blend with another ground. In the picture you can see, there are two power supply that is VCC1 and VCC2, VCC1 is devoted only to the Thermocouple where ground VCC1 (GND) should not be fused with the another ground, GND is ground only for a series of ADG507A, AD595 and INA126. While VCC2 with ground GNDA is ground that can be associated with other equipment such as ground on data acquisition board.

HCNR201 is high-linearity analog optocoupler consists of a high-performance AlGaAs LED that illuminates two closely matched photodiodes PD1 and PD2, as shown in Figure 1. The input photodiode PD1 can be used to monitor, and therefore stabilize, the light output of the LED. As a result, the on linearity and drift characteristics of the LED can be virtually eliminated. The output photodiode PD2 produces a photocurrent that is linearly related to the light output of the LED. The close matching of the photodiodes and advanced design of the package ensure the high linearity and stable gain characteristics of the optocoupler.

The HCNR200/201 can be used to isolate analog signals in a wide variety of applications that require good stability, linearity, bandwidth and low cost. The HCNR200/201 is very flexible and, by appropriate design of the application circuit, is capable of operating in many different modes, including: unipolar/ bipolar, ac/dc and inverting/ noninverting. The HCNR200/201 is an excellent solution for many analog isolation problems.

In the above circuit set the VR3 to obtain your desired voltage according to the thermocouple output gained .

Source :

1. Complete circuit of Low Cost (8-Channel) Thermocouple Signal Conditioning With Analog Multiplexer and Analog Optocoupler.
2. Datasheet ADG507A
3. Datasheet AD595
4. Datasheet INA126
5. Datasheet HCNR201
6. Thermacouple Singnal Conditioning PCB (Bottom Track).zip
7. Thermacouple Singnal Conditioning PCB (Solder Mask).zip
8. Thermacouple Singnal Conditioning PCB (TOP Legend).zip

The author is not responsible for any risk caused by this circuit. Refer all to the datasheet.