LTC2487

Hi JCCillion ,

Can you please help look into this query?
Thanks.

Regards,
Francis

Hello Francis-san, JCCillion-san

Francis-san,
Thank you for forwarding it.

JCCillion-san
Thank you for your support. I'd like your some help.

Hello,
Sorry if this is difficult to understand. This question is about the input resistance value of CH2 to COM or CH3 to COM.
Is it over 100GΩ, similar to a zero-drift amplifier?
If it is low impedance, I would appreciate it if you could suggest your recommend a zero-drift amplifier that would fit the input of this ADC. Is there any other information I need?
If the customer is looking for precision and needs an amplifier, may I how about suggest the ADA4528-1/2 or LTC2057?

Hi S1640 ,

I am currently looking into your inquiry. I will come back to you with an answer soon.

Thank you for your patience.

Regards,

JC

I apologize for the inconvenience. I will change the confirmation contents.

Regarding Fig. 38 in the LTC2487 datasheet,
(1) The source resistance of the CH1/CH2 input circuit is up to 100kΩ, but the APPLICATIONS INFORMATION on page 25 of the datasheet states that resistances up to 10kΩ can be connected directly to the LTC2487. What does this mean?

(2) Are there any problems with using the ADA4528-1/2 as a replacement for the LT1494?

(3) Is it possible to use a differential amplifier or instrumentation amplifier instead of a single-ended buffer configuration? Are there any other solutions if higher precision is desired?

Hi S1640 ,

The LTC2487 does not specify the input impedance because the input impedance is dynamic due to the switched-capacitor. The datasheet specifies the leakage current of +/-10 nA, which an input voltage of 5V can translate to a DC impedance of ~500 Megaohm (5V/10nA). It may not be as high as 100Gohm, but it should be able to directly interface high-impedance sensors. May I know what the typical impedance of the input signal is?

If the input impedance is low, then an amplifier would not be necessary. With a 1kohm input resistance, it should be fine without a front-end amplifier. If the COM pin is connected to ground, a buffer would be sufficient

Regarding your questions,

S1640 said:

(1) The source resistance of the CH1/CH2 input circuit is up to 100kΩ, but the APPLICATIONS INFORMATION on page 25 of the datasheet states that resistances up to 10kΩ can be connected directly to the LTC2487. What does this mean?

What is mentioned in page 25 of the datasheet is the impedance of the reference when an external reference is used. The datasheet mentions that a reference impedance up to 10kohm could use the same Cref = 100pF without performance degradation.

S1640 said:

(2) Are there any problems with using the ADA4528-1/2 as a replacement for the LT1494?

The ADA4528-1 offers a better input bias current. A low input bias current should be used to avoid creating offsets across the source impedance. 

S1640 said:

(3) Is it possible to use a differential amplifier or instrumentation amplifier instead of a single-ended buffer configuration? Are there any other solutions if higher precision is desired?

Additional amplifiers may be added if higher precision is desired. I am not very knowledgeable in terms of amplifiers but you could post your amplifiers inquiries in this forum. (+) Amplifiers - EngineerZone

Regards,

JC

Hi JCCillion-san,

Thank you for your quick response.

Is this mean ? "If the input impedance is low, then an amplifier would not be necessary. With a 1kohm input resistance, it should be fine without a front-end amplifier. If the COM pin is connected to ground, a buffer would be sufficient"

Additional confirmation.

(1) The source resistance of the CH1/CH2 input circuit is up to 100kΩ, but the APPLICATIONS INFORMATION on page 25 of the datasheet states that resistances up to 10kΩ can be connected directly to the LTC2487. What does this mean?

 What is mentioned in page 25 of the datasheet is the impedance of the reference when an external reference is used. The datasheet mentions that a reference impedance up to 10kohm could use the same Cref = 100pF without performance degradation.

→Thanks, Would anything over 100kΩ be a problem?

(3) Is it possible to use a differential amplifier or instrumentation amplifier instead of a single-ended buffer configuration? Are there any other solutions if higher precision is desired?
Additional amplifiers may be added if higher precision is desired. I am not very knowledgeable in terms of amplifiers but you could post your amplifiers inquiries in this forum. (+) Amplifiers - EngineerZone

→Does this mean that it is possible to add a differential amplifier or an instrumentation amplifier?

Best Regards,

Hello, JCCillion-san

Thank you for your support.

Is it okay to connect a thermocouple directly to an Easy Drive product such as the LTC2478?
What I confirmed you previously was not a thermistor but a thermocouple.
I would like to hear your opinions on combining a thermocouple with the LTC2478.

Thank you.
Best regards,