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Sangeeta Kumar

(From: College of Engineering, Pune; To: Texas Instruments, Bangalore)

Journal Publications

Conference Publications

Thesis

Title: Low-Distortion, High-Q Bandpass Filter For Measurement Of High-Resolution ADCs

The desire to achieve high resolutions has seen the advent of 18-bit and higher resolution SAR-ADCs. The distortion of the source required in testing linearity of the SAR-ADC should be significantly lower than the ADC itself. Hence, a sine wave with a total harmonic distortion of −120 to −140dB is required to measure 18 to 20 bit ADCs.

Since the ADC’s resolution is comparable to noise, static parameters such as INL and DNL of the ADC is obtained by averaging over several cycles of the input. Since ADCs are tested at audio frequency, this may be over several hundreds of milliseconds. Hence, the amplitude and frequency of the source must be stable over the test period. These are the two basic requirements from the source that is used to test high-resolution ADCs.

Analog function generators give the required distortion performance. However, the RC time constant of such a generator varies over time and hence their amplitude and the frequency drift over time. To have a better frequency and amplitude stability, DAC-based sine wave generators can be used. However, their distortion is limited by matching of the DAC elements.

The amplitude and frequency drift of the analog generator are inherent to the design of the oscillator itself and hence will be difficult to rectify. On the other hand, it is easier to filter out the distortion of a DAC-based generator. Hence, a low-distortion bandpass filter is required to filter the distortion of a DAC-based signal generator.

Bandpass filters can be built using discrete components. However, their performance will be limited by the matching of the components used which will, in turn, affect the RC time constant of the filter. Hence, an integrated bandpass filter is desired, which will filter the harmonics arising from a non-precision DAC-based source generator to produce a clean sine wave that has a stable frequency and amplitude over time.

Another advantage of having a bandpass filter is that it is cheap and portable, whereas the function generators are expensive and bulky. Hence, the number of devices that can be tested parallelly is limited by the number of function generators available, whereas this is not the case for an integrated bandpass filter.

This work describes the design of such a bandpass filter. Since a large attenuation is required at the second and the third harmonic frequencies, active filters are to be used. Active filters use transistors which themselves contribute to distortion. In this work, distortion mechanisms in active filters are analyzed and methods are proposed to reduce the distortion at the output of the filter. A two-stage Miller-compensated opamp is considered and various sources of distortion are discussed. The nonlinearity of the output stage of the opamp used in the active filter coupled with the capacitance at the input of that stage is the main cause of nonlinearity. Using cascoded Miller compensation along with a buffer inserted between the first and second stage of the opamp helps suppress this nonlinearity.

Two filter prototypes with 1kHz and 10kHz center frequency and a second harmonic attenuation of 60dB are realized using a cascade of four second-order bandpass sections. Each second-order bandpass section is made of Tow-Thomas biquads with a quality factor of 4. Each filter is fabricated in a 0.6µm process and occupy an area of 11mm2 and consumes 65mW from a 5.6V supply.

The measured THD (median of measurement from 10 chips) and output noise for a 10Vppd output is −115dB and 70µVrms for the filter with 1kHz center frequency and −119dB and 71µVrms for the filter with 10kHz center frequency. Such levels of distortion in a 10 Vppd signal enables us to measure the linearity of 18-bit ADCs.