Why matching is needed

[VLSI_Learner]

Why matching is needed in analog layout?

 

[Dominik Przyborowski]

To avoid offset in differential pairs, to provide DAC/ADC resolution, accurate current sources, etc...
Think about mismatch as a dc noise

 

[jiripolivka]

In general we want all power from the source to get to the load. Matching means that source and load resistance should be equal (for sinusoidal signals, conjugate) to meet the best transmission condition.

 

[VLSI_Learner]

Can anyone please elaborate, more explanation needed, please. Thank you.

 

[Dominik Przyborowski]

In one of your previous thread I left You names which You should put into google... But OK.

The two types of mismatches exists - statistical related to fluctuation of device dimensions, gate oxide thickness, dopants - and systematic caused by gradients of dopants density, polysilicon and SiO2 thickness, asymetry of drain/source regions caused by nonorthogonal implantation, etc. To systematic mismatches we also including "bad designs", what means layout of cell made without care about it, e.g. different metal coverage of devices, rotating one relate to other, different metal paths resistance seing from source node of transistors, etc.

All of them causing differences in current gain factor and threshold voltages of "theoretical" identical transistors or resistance value for resistors.

We need good matching for precision circuits like D/A and A/D converters or differential amplifiers. For example if You will designing current steering DAC with 8 bit resolution, to obtain INL values below 0.5LSB and then ENOB >7.5bit with 99% of yield your current source for MSB bit should meet following requirement:
\[3 \sigma(I_{MSB}) \leq \frac{100 \%}{2^8} \simeq 0.39 \% \Rightarrow \sigma(I_{MSB}) \leq 0.13 \%\]
The above condition including only statistical effects. Usually we haven't information about gradients of oxide thickness, etc. (sometimes but very rarely its possible to find in process documentation distance matching parameters) so we using special layout techniques to avoid systematic mismatch sources.
For example, for this 8 bit DAC a common/double centroid layout techniques with at least one dummies ring should be used.

For last 32 years peoples wrote many articles and thesis about matching, also a few books seriously discusses this topic.

 

[kenambo]

Matching is used to reduce the effects of systematic mismatches like doping density, ... in X or Y direction OR X and Y direction....

1]Interdigitisation 2]common centroid techniques used as famous and effficient matching techniques....
Matching reduces the non linearites in the path of the signal.... especially in diff amplifiers DAC and ADC ... which gives reliable outputs which is reasonable to the schematic test results..

Thanks...