Matching Technique in Analog Layout
In Analog circuits, designs are used like differential pairs and current mirrors, where the matching of device characteristics such as the threshold voltage Vt is important. If device threshold differences of a few millivolts or less can determine the difference in performance and yield of a design.
Threshold voltage also varies due to the variations in the number of doping atoms in the channel and also it reduces with decreasing gate oxide thickness. For that, we need to match the analog devices
Two Matching Techniques for Analog Layout
- Common Centroid
The main object of Common Centroid is that the centroids of each device should coincide, the devices should be symmetrical, their orientation should be the same, and the array should be as compact as possible
The four rules of Common Centroid:
There are a lot of ways of combining common centroid and Here is some example for Common Centroid:
The common centroid technique is very good at reducing the effect of thermal or linear process gradients and may be present in an integrated circuit. also, this technique distributes the effect of gradient more evenly among the devices
In an interdigitated pattern, all the transistors are in an interleaved pattern like suppose there are two transistors A & B with 2 fingers each. then ABAB or ABBA or AABB called interdigitation.
Here the first half is exactly mirrored and the variations for A and B are the same. While in a common centroid pattern we care for X and Y variations unlike in interdigitated we care only X variations while in common centroid we can match in both axis ( X & Y-axis).
Reasons of Mismatch
Mismatch in integrated circuits are generally of two types :
- Random mismatches
- Systematic mismatches
- Random mismatches due to microscopic fluctuations in dimensions, doping, oxide thickness, and other parameters that influence component values
- Systematic mismatches are caused by: Process biases, Mechanical stress, Temperature gradients, and Polysilicon etch rates, etc.
Rules for MOS transistor matching
- Place transistor segments in the areas of low-stress gradients.
- Place transistors in close proximity.
- Orient transistors in the same direction.
- Keep the layout of the transistors as compact as possible
- Whenever possible use Common centroid layouts.
- Place transistors well away from the power devices.
- For current matching keep the overdrive voltage large.
- For voltage, matching keeps overdrive voltage smaller.
Matching techniques in analog layout design are employed to ensure that components or devices within the circuit have similar characteristics or behaviors. This is important in analog circuits as small variations in devices can result in performance degradation or deviation from the desired specifications. Here are some commonly used matching techniques in analog layout design:
- Common-Centroid Layout: In this technique, multiple identical devices are placed in a symmetric and interdigitated manner to minimize mismatch caused by process variations. By placing the devices in a common centroid pattern, any process-induced variations affect all devices equally, resulting in improved matching.
- Interdigitated Structures: Interdigitated structures are used to distribute variations across multiple units of a device, such as transistors. By interleaving the fingers of multiple transistors or devices, the impact of process variations is spread out, reducing mismatch.
- Layout Symmetry: By using symmetrical layout structures, devices can be placed in a way that minimizes process variation effects. This includes maintaining symmetry in device shapes, gate layouts, metal traces, and routing.
- Dummy Devices and Fills: Incorporating dummy devices or dummy metal fills in the layout helps to match the parasitic capacitance and resistance of devices. By including these dummy structures, the overall matching performance can be improved.
- Proximity-Aware Layout: Placing devices in close proximity to each other helps to minimize the effects of process variations. This technique reduces the impact of spatial variations and improves matching.
- Current Density Constraints: By constraining the current density across layout structures, mismatches caused by variations in resistive elements can be minimized. This involves ensuring that the current density is the same for matched devices.
Applying these matching techniques in analog layout design helps to ensure consistent performance and improved matching capabilities in analog circuits.
How does a common-centroid layout differ from an interdigitated layout in terms of resistor matching?
A common-centroid layout improves resistor matching by sharing a common center between resistors, ensuring that their parasitics are uneven but their values are closely matched. This layout is more effective in achieving matching compared to an interdigitated layout.
In what other electronic components or devices can common-centroid layouts be used to improve matching, and why may their effectiveness diminish with smaller process technologies?
Common-centroid layouts can also be used to improve matching in MOSFETs or capacitors. However, their effectiveness diminishes in smaller process technologies because defects in smaller devices tend to dominate their behavior. In such cases, using larger areas is a more effective technique to improve matching, although common-centroid techniques can still offer some minor improvement when layout sizes are larger
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