A current-mode testable design of operational transconductance amplifier-capacitor filters

Analog filters are important building blocks of many communication and instrumentation systems. However, similar to other analog circuits, testing an analog filter is a difficult problem. In recent years, this problem has become even more difficult because of the increase of circuit complexity. Operational transconductance amplifier-capacitor (OTA-C) filters are especially useful in video applications such as HDTV. In this paper, we first present the design of an OTA-C low-pass filter with a passband from 0 to 4.5 MHz. We then propose a current-mode method for the error detection of this filter. By comparing the current consumed by the circuit under test (CUT) and the current converted from the voltage levels of the CUT, abnormal function of circuit components can be concurrently detected. This technique has been applied to the OTA-C filter and a testable design is obtained. Experimental results show that our design has the following advantages: (1) easy to design and implement; (2) high accuracy in error detection; (3) little impact on the circuit performance of the filter; and (4) high error-detection speed. From an actual layout, we find that the area overhead is about 25% and only one extra pin is needed.