LM224A: Which parameter can impact output voltage?

Jimmy WANG

Part Number: LM224A
Other Parts Discussed in Thread: OPA4990, TLV9354, TLV9304

Hi team,

Customers use LM224A to detect voltage. but found some issues here.

LM224A amplifies different voltages respectively. The normal output voltage is 1.65V, but the abnormal voltage output is about 1.53~1.55V.
They suspect there is something wrong with the IC. But according to my understanding. Vos_max is 3mV. When the gain is 9
The maximum increase or decrease is 27mV. Why does it exceed the set value?

schematic as below.

over 1 year ago

0 Clemens Ladisch over 1 year ago

Guru 296410 points

This schematic is incomplete; what are the power supply voltages? What are the voltages at the I/O pins?

0 Ashley Kang over 1 year ago in reply to Clemens Ladisch

TI__Genius 13570 points

Hi Jimmy,

Along with Clemens' questions, could you clarify if you are seeing this issue with all four channels of the op amp? How is the fourth channel of the op amp connected?

If you could provide an oscilloscope image of the inputs and outputs of the op amp to see what is happening, that would be helpful too.

Thank you!
Best Regards,
Ashley

0 Jimmy WANG over 1 year ago in reply to Ashley Kang

Genius 3615 points

Hi Ashley,

all channels as below.

VCC=16.5V

TP31=1.83V

but abnormal channels are possible in BCD one of the channels.

The abnormal voltage range is 1.53~1.55V. customer channel OP will fix the issue. Is it an error range?

0 Clemens Ladisch over 1 year ago in reply to Jimmy WANG

Guru 296410 points

1.83 V × 9 = 16.47 V. But the LM224A does not have rail-to-rail outputs; the V_OH specification shows that even a small output current requires a headroom of 1.5 V to the positive supply.

You could change the amplification factor so that the output will not go above 15 V. Alternatively, use an opamp with rail-to-rail outputs like the TLV9304, TLV9354, or OPA4990.

0 Jimmy WANG over 1 year ago in reply to Clemens Ladisch

Genius 3615 points

Hi ,

Sorry, TP31=0.183V Gain=9

Basically, the OP's BCD channel output is about 1.65V
But the abnormal channel is 1.53~1.55V. Customers want to know why.
Because replacing an IC can solve the problem

0 Clemens Ladisch over 1 year ago in reply to Jimmy WANG

Guru 296410 points

Are the voltages at the two input pins (5/6 etc.) exactly the same? What is the voltage drop over the 100 Ω resistor?

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LM224A: Which parameter can impact output voltage?