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Can AD8221BR, AD8622 improve current design?

ksathyadev
on Jun 7, 2016

We are redesigning a test board due to heating in a few LT1167 (LT1167ACS8) and LT1352 (LT1352CS8) devices. Due to lack of information on the specific devices that heat up, done LT Spice simulation for circuits, and have nearly narrowed down on the problem.

LT1167 analysis: In simulation seen that Is(source current) is 2.1mA, whereas datasheet for the device gives 1.3mA (maximum). So, it is probable that the LT1167 devices get heated up when the specific test is run for an extended duration. This LT1167 is used with gain of 6.18, others with unity gain.

Here we are planning to replace LT1167 with AD8221BR. Primary criteria for selection of Instr Amp was:

a) Input bias current (max) & input offset current (max) to be < 400pA

b) -3dB bandwidth not to go far below 800kHz

c) CMRR to meet or exceed 90dB/min to 95dB/typ (G=1), and 106dB/min to 115dB/typ (G=10)

LT1352 analysis: This is used to implement unity gain amplifier at output of LT1167, and at other places as low pass filter (fc=100Hz). In simulation seen two issues:

a) Device current per Op Amp stays at 0.45mA for a test case, which is more than the 250uA/min to 320uA/typ limit of LT1352. At this instant, Op Amp output is 6.9V (nearly half the supply voltage, 15V)

b) For another test case, seen that there are -15V to +7V voltage swing signals on +/- input and output lines of LT1352, though there is no differential between the +ve and -ve inputs. The datasheet only mentions that the device is not suited for continuous high swing differential inputs.

Here we are planning to replace LT1352 with AD8622.

Queries to the forum are:

1) For AD8221BR, can we consider that VOS = VOSI + (VOSO/G) would hold true with VOSI=25uV and VOSO=200uV (values from AD8221BR datasheet, page4).

2) Would there be heating on AD8622 as well, with this device also having a supply current of 175uA - 225uA. Yet to do simulation which should give insights.

3) Does Op Amp heating primarily happen when Is(supply current) is at the maximum and with output biased at half the VCC or Vee supply voltage

Thank you.

Regards

-Sathyadev

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    •  Analog Employees 
    on Jun 9, 2016 2:21 AM

    Hi Sathyadev,

    Thank you for considering ADI products on your applications.

    Here are my answers to your questions.


    1. The VOSI and VOSO parameters specified on the AD8221 datasheets are both tested in production and therefore we can guarantee its value. The equation that you provided is the total offset voltage (referred to the input) due to to input offset voltage (VOSI) and output offset voltage (VOSO) only. Note that there are also other components that could introduce DC error like the input bias current (IB) and  input offset current (IOS) together with the unbalanced source impedance, Common Mode Voltage, Power supply variation, etc.

    2. Please refer to this article for the computation of the junction temperature of the device: http://www.analog.com/media/en/technical-documentation/technical-articles/MS-2251.pdf . Note that if you are going to use the AD8622, make sure that the opamp is operating within its input and output voltage range. If you could share to me your specific setup or configuration then we could analyze it in more detail.

    3. By referring on the article above, we could compute and verify if the heating of the parts is expected or there might be wrong connections on your setup.

    Let me know if you need anything else.

    Best regards,

    Emman

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  • 0
    •  Analog Employees 
    on Jun 9, 2016 2:21 AM

    Hi Sathyadev,

    Thank you for considering ADI products on your applications.

    Here are my answers to your questions.


    1. The VOSI and VOSO parameters specified on the AD8221 datasheets are both tested in production and therefore we can guarantee its value. The equation that you provided is the total offset voltage (referred to the input) due to to input offset voltage (VOSI) and output offset voltage (VOSO) only. Note that there are also other components that could introduce DC error like the input bias current (IB) and  input offset current (IOS) together with the unbalanced source impedance, Common Mode Voltage, Power supply variation, etc.

    2. Please refer to this article for the computation of the junction temperature of the device: http://www.analog.com/media/en/technical-documentation/technical-articles/MS-2251.pdf . Note that if you are going to use the AD8622, make sure that the opamp is operating within its input and output voltage range. If you could share to me your specific setup or configuration then we could analyze it in more detail.

    3. By referring on the article above, we could compute and verify if the heating of the parts is expected or there might be wrong connections on your setup.

    Let me know if you need anything else.

    Best regards,

    Emman

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