The effective capacitance of ceramic capacitors typically decreases with added DC bias.
DC bias dependency is quite well-known and documented in capacitor datasheets. It has a clear relationship with capacitor density. There is a tendency for small packages with high marked nominal capacitance to degrade quickly when DC bias is applied. It might not become a significant problem in low-voltage supplies such as 1.3 V, but it can have a remarkable impact at 3.3 V.
The following graph shows the DC bias curves for the capacitors. The graph is created using the free Murata SimSurfing tool.
Figure 1. Typical capacitance changes vs. DC bias
The following table lists the Murata general purpose capacitors that are evaluated in terms of DC bias:
| Line nr | Part number (graph color) | Value | Type/Size | Voltage | Capacitance change at 3 V |
|---|---|---|---|---|---|
| 1 | GRM155R61A104KA01 (cyan) | 100 nF | X5R/0402 | 10 V | −3% |
| 2 | GRM155R61A105KE15 (green) | 1.0 μF | X5R/0402 | 10 V | −20% |
| 3 | GRM155R61A475MEAA (blue) | 4.7 μF | X5R/0402 | 10 V | −30% |
| 4 | GRM155R70J105KA12 (red) | 1.0 μF | X7R/0402 | 6.3 V | −40% |
- The density of the X5R 100 nF capacitor is fair compared to that of the 1 μF and 4.7 μF capacitors, and therefore DC bias does not influence effective capacitance on the low end of the voltage range.
- The X5R 1 μF capacitor has a moderate effect on DC bias and would present almost all of the capacitance at 1.3 V but degrades to −20 % at 3 V bias.
- The X5R 4.7 μF capacitor could be considered for decoupling higher voltage pins. However, the tool reveals that effective capacitance is reduced by 30 % because of the DC bias dependency at 3 V.
- The X7R 1.0 μF capacitor has the highest DC bias dependency. It could still be considered a capacitor to decouple lower voltage pins operating at 1.3 V, for instance, as the degradation is less than 10 % at this low voltage.