cn-0209

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

Hi,

     Thanks for the quick response.I need help regarding the following:

1. The voltage from the thermocouple will be small(in mV or uV range).Then how is it possible to directly connect the thermocouple voltage to the ADC without any signal conditioning?
2. Can you make it more clear about how the ±10V signal is fed to the ADC?
3. In the circuit, is AD8676 and AD8275 meant for RTD alone?
4. Can you help me regarding the selection of microcontroller for the Programmable Analog Front End?

Regards,

Divya

Hi Divya,

In response:

1. I should have mentioned, that AD7193 includes a precision PGA (gain of 1 to 128). This PGA provides a high impedance input for the thermocouple, as well as gaining up the thermocouple signals to utilise more of the ADC range.
2. The ±10V signals are passed through the ADG1414 switch and are deferentially buffered by the AD8676 to provide a high input impedance. The ADG1414 and AD8676 have ±15V supplies to allow for the ±10V at their inputs. The output of the AD8676 is fed into the AD8275, which performs the level shifting and attenuation (Gain = 0.2) to the range of the ADC.
3. No, the RTD input would also be connected directly to the AD7193 via the ADG442. The AD8617 provides the excitation current.
4. This really Depedns on your requirements:
   • You can purchase the CN0209 hardware and an SDP board. The SDP board is a controller board with interface to the PC, it contains a DSP on board. www.analog.com/sdpb
   • Another option is to use the ADuCM361, this is a cortex M3 core with precision 24-bit sigma delta, Mux and PGA. So this would replace the AD7193, and includes the microcontroller. Note that this is a 3.3V devices so the circuit would need to be altered accordingly.
   • Alternately any other micro-controller of your choice. The AD7193 has an SPI compatible interface.

Regards,

Derrick

Hi,

    That was a very helpful piece of information.Thanks.

1. If I select voltage signals for both channel1 and channel2 ,the voltage will pass through the ADG1414 and then to AD8676.But in this case ,won't the voltages overlap?
2. I want to modify the circuit to design a four channel Universal Analog Input Module.For this ,is it advisable to duplicate the components used(ADG1414,ADG442,AD8617,AD8676,AD8275)?

Regards,

Divya

Hi Divya,

The front end will only sample one channel at a time. The procedure would be as follows:

1. Sample channel 1
   1. Close S5, S7 of ADG1414, all other switches open
   2. Allow input to settle
   3. Measure input channel
2. Sample channel 2
   1. Close S6, S6 of ADG1414, all other switches open
   2. Allow input to settle
   3. Measure input channel
3. Repeat

If both channels were thermocouple, the procedure would be similar, though you'd be using he ADG442 switches instead.

If Channel1 was ±10V and Channel2 a thermocouple, then it would be possible to leave Channel 1 connected to the AD8676 and Channel2 connected directly to the AD7193. Then use the AD7193's internal Mux to switch between them.

To support 4-channels there are two options:

1. Add another copy of the ADG1414 circuitry (including AD8617); note that the voltage input for this can go to the same AD8676 as already present in the circuit. Also Add another ADG442 switch for the thermocouple/RTD; I would switch all the thermocouple RTD's to AIN1 and AIN2. The reason for this is that you can then add another 2 current inputs and connect the 200ohm resistors to AIN5 and AIN6 respectively.
2. You could double up the entire circuit (including ADC). The advantage of this is that the sample rate per channel will not decrease over the two channel version; the more channels you add into the ADC, the slower your effective sampling rate as you need to switch+settle between more channels.

Regards,

Derrick