The Texas Instruments (TI) ADS809 is a 12-bit, 80-MHz pipeline analog-to-digital converter (ADC). It has high speed, high resolution, high-input bandwidth, and a high signal dynamic range. Its many other good features include good signal-to-noise-ratio (SNR), good linearity, low jitter, flexible clocking, an over-range indicator (OVR), “data valid” output, three-state output, an internal or external reference, and a single-ended or differential input configuration. It can be used for broadband communications, test equipment, medical instrumentation, CCD imaging, and other fast-ADC applications.
In these applications the input analog signal applied to the ADS809 varies and may be a DC, AC, narrow band, wide band, or pulse signal with a large amplitude. The sampling clock used for these applications can be up to 80 MHz with a uniform or non-uniform clock phase. Different applications require different critical features of the ADS809. In some conventional sampling applications, the ADC input analog signal is smooth with a large amplitude, and the SNR and spurious-free dynamic range (SFDR) are critical; while in some small-signal sampling applications, the SFDR and full-scale (FS) step-acquisition time are not as critical. However, the FS step-acquisition time is critical in largesignal sampling applications, especially when the sampling clock has to be high-frequency with a non-uniform phase and the analog input pulse signal is large (for example, 2 V). In this case the input signal has a sharp edge with a large voltage amplitude, and the pulse edge could be very close to the sampling clock edge due to sampling clock phase variations. If the ADC does not have fast step response for the signal settling, the next ADC sample after the pulse edge will be unstable, which is undesirable. Therefore it is critical for the ADC to have a fast settling time when a large input pulse signal is sampled. Similarly, when the input pulse is over full-scale range (FSR), it is important for the ADC to have a fast over-range recovery time. This is tough for any ADC in applications where the input pulse signal is large and sampling speed is very high with a nonuniform clock phase. However, the ADS809 works well with this type of application. This article presents the results of recent lab tests that further prove the ADS809’s fast step response with a large, FS input pulse. The test data also covers the ADS809’s response to over-range conditions.
