Can AD8221BR, AD8622 improve current design?

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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  • Hi Emman,

    We filtered out the Op Amps which can be a suitable replacement, and selected AD8512 (B grade, dual Op amp part) for the design. In simulation we put AD8512 (single Op amp part) as parameters are same.

    Observations for Icc on both Instr Amp AD8221 and Op Amp AD8512 are as follows:

    a) Icc for Instr Amp AD8221 is within limits

    b) Current on Vcc pin of AD8510 is in the range of a few 100mA in negative, and a cause for concern. Could you help isolate the issue, if with model or circuit?

    c) Current on Vee pin of AD8510 is < 2.4mA. The Icc limit on AD8510 is 2mA (typ), 2.3mA (max).

    we are working with limited information on the specs or the reason for heating on the existing LT1167 and LT1352 devices hence trying one option after another. Since we saw higher Icc on LT1167 and LT1352 devices, ran this simulation, and it has helped on Instr Amp.

    The load on the line COM1 is implemented using LT1112 Op Amps.

    Any suggestions?

    Attached zip which includes the AD simulation circuit and Op Amp selection sheet.

    Regards

    -Sathyadev

    2_NGAIS_AO1-AO3_Jun28.zip
Reply
  • Hi Emman,

    We filtered out the Op Amps which can be a suitable replacement, and selected AD8512 (B grade, dual Op amp part) for the design. In simulation we put AD8512 (single Op amp part) as parameters are same.

    Observations for Icc on both Instr Amp AD8221 and Op Amp AD8512 are as follows:

    a) Icc for Instr Amp AD8221 is within limits

    b) Current on Vcc pin of AD8510 is in the range of a few 100mA in negative, and a cause for concern. Could you help isolate the issue, if with model or circuit?

    c) Current on Vee pin of AD8510 is < 2.4mA. The Icc limit on AD8510 is 2mA (typ), 2.3mA (max).

    we are working with limited information on the specs or the reason for heating on the existing LT1167 and LT1352 devices hence trying one option after another. Since we saw higher Icc on LT1167 and LT1352 devices, ran this simulation, and it has helped on Instr Amp.

    The load on the line COM1 is implemented using LT1112 Op Amps.

    Any suggestions?

    Attached zip which includes the AD simulation circuit and Op Amp selection sheet.

    Regards

    -Sathyadev

    2_NGAIS_AO1-AO3_Jun28.zip
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