The old National Semi numbering system was LM1xx for Mil-temp devices, LM2xx for industrial temp, and LM3xx for commercial temp. Then the LMyx1 was for the single version, LMyx2 for dual and LMyx4 for the quad.?
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The LM121 I am looking for is unique as it has a differential output and is paired with another opamp such as the LM118 to become a low-drift, high-gain set.
DerekK