Testing of embedded A/D converters in mixed-signal circuit

In this paper, a complete functional testing of embedded ADC is presented. The integral non-linearity error, INLE, differential non-linearity error, DNLE, offset error, OSE, gain error and the signal-to-noise ratio, SNR are tested. The problem related to the propagation of the analog signal to the input of the ADC and the observation of the digital output of the converter at the output of the digital circuit are discussed. The two means for functional testing are discussed: the histogram and he FFT. To observe the outputs of the ADC at the digital circuit, the inverse function of the digital circuit is computed. This can be done by inverting the transfer function of the digital circuit whenever it is available. In the other case, when the digital circuit structure is available, the inverse of the digital circuit is found by boolean function manipulation. Il. Introduction In order to present a complete test for an ADC embedded in a mixedsignal circuit, some performances have to be measured at the output of the mixed-signal circuit. These performances are: the integral non-linearity error INLE, differential nonlinearity error, DNLE, offset error, OSE, gain error, GE, the signal-to-noise ratio, SNR and the effective number of bits, KNOB [l]. They are measured by different means: the histogram; which is appropriate for DNLE [I] measurement, the FFf technique, which is used for SNR measurement [3], the static performance measurements which are performed at DC level. INLE, DNLE, GE and OSE can be considered as static as well as dynamic performances, whereas the SNR is a dynamic performance. The main contribution in this paper is the complete functional test of an embedded ADC without circuit modification. In fact, up to now a mixed circuit has been partitioned into digital and analog subcircuits in the test mode. It was therefore considered that the ADC inputs and outputs are accessible. Since it is not always possible to modify the circuit, due to performance degradation and pin-count limitations, we have to test a mixed circuit as an entity and which is without modifications. Thus, this work is an extension of the work presented in [2], where we introduced a technique for structural testing of analog and digital blocks of a mixed circuit without circuit modification. Here, our objective is to propose an approach for complete testing of an embedded ADC without any modification. 2. Functional testing of embedded ADC The most famous methods for dynamic testing of ADC’s [S] are the code density histogram method and the spectral analysis with FFI. Histogram data are used to compute all bit-transition levels, and hence determine linearity, gain, and offset errors [S]. FFf data are used to characterize the linearity and noise properties of the ADC in the frequency domain. The two methods are useful in measuring the ADC DNLE and INLE using a sine wave with unknown amplitude and offset. The FFT method alone is used to compute the signal-to-noise ratio (SNR) and the effective number of bits. Some attempts have been made to use either the histogram method or the FFI method alone to fully characterize the ADC. All the classical techniques for ADC testing and diagnosis can be appiied to an embedded ADC testing. But first we have to consider the propagation of the analog signal through the analog subcircuit and observation of the digital code at the output of the digital circuit. The propagation of the analog signal to the input of the ADC is performed easily. In fact, since the subcircuit under test is the ADC, all the remaining subcircuits are considered to be fault free. For digital code propagation from the ADC outputs to the outputs of the digital subcircuit, we need to find the inverse function of the digital subcircuit. This inverse function can be found either by boolean function manipulation, when only tbe structure of 1063-6404/96 $5.00