cn-0209

Hi,

      I need to design a universal analog input module .I have referred to CN-0209.But I could not understand the purpose of ADG442 used in the circuit.Can I get help?

Regards,

Divya

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  • 0
    •  Analog Employees 
    on Mar 4, 2014 1:29 AM

    Hi Divya,

    For quick reference, here is a link to the circuit note: http://www.analog.com/cn0209

    In CN0209, the purpose of ADG442 is to allow for high precision thermocouple measurements.

    • In thermcouple input mode the thermocouple signal is switched directly to the ADC input using the ADG442. This bypasses the AD8676 and AD8275 (which would add noise as well as offset/gain errors to the small thermocouple signal)
    • So why not connect directly to the ADC without using a switch:
      • In voltage input mode, ±10V signals will be present at the input. The ADC cannot tolerate these high voltage on it's inputs (-0.3V to 6.5V absolute max for AD7193).
        • Thus the ADG442 will stand off the ±10V signals in voltage input mode.
      • In thermocouple mode the switches will pass the low voltage thermocouple signals directly to the ADC inputs. This allows for a high precision thermocouple measurement.

    One other thing to mention is that in thermocouple mode there is a 2.25V bias voltage generated for the thermcouple by the AD8617 in the circuit,

    Another universal input reference circuit that may interest you is CN0325: www.analog.com/cn0325. This is a single channel universal input solution, and allows for a fully configurable input with 4 terminal blocks (like CN0209), or a lower cost option using 6 terminal blocks.

    Regards,

    Derrick

Reply
  • 0
    •  Analog Employees 
    on Mar 4, 2014 1:29 AM

    Hi Divya,

    For quick reference, here is a link to the circuit note: http://www.analog.com/cn0209

    In CN0209, the purpose of ADG442 is to allow for high precision thermocouple measurements.

    • In thermcouple input mode the thermocouple signal is switched directly to the ADC input using the ADG442. This bypasses the AD8676 and AD8275 (which would add noise as well as offset/gain errors to the small thermocouple signal)
    • So why not connect directly to the ADC without using a switch:
      • In voltage input mode, ±10V signals will be present at the input. The ADC cannot tolerate these high voltage on it's inputs (-0.3V to 6.5V absolute max for AD7193).
        • Thus the ADG442 will stand off the ±10V signals in voltage input mode.
      • In thermocouple mode the switches will pass the low voltage thermocouple signals directly to the ADC inputs. This allows for a high precision thermocouple measurement.

    One other thing to mention is that in thermocouple mode there is a 2.25V bias voltage generated for the thermcouple by the AD8617 in the circuit,

    Another universal input reference circuit that may interest you is CN0325: www.analog.com/cn0325. This is a single channel universal input solution, and allows for a fully configurable input with 4 terminal blocks (like CN0209), or a lower cost option using 6 terminal blocks.

    Regards,

    Derrick

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