ADC
AS1419
AS1419A
Austin Semiconductor, Inc.
then the AS1419 inputs can be driven directly. As source
impedance increases so will acquisition time (see Figure 6). For
minimum acquisition time with high source impedance, a buffer
amplifier should be used. The only requirement is that the
amplifier driving the analog input(s) must settle after the small
current spike before the next conversion starts (settling time
must be 200ns for full throughput rate).
Intermodulation Distortion
If the ADC input signal consists of more than one spectral
component, the ADC transfer function nonlinearity can pro-
duce intermodulation distortion (IMD) in addition to THD. IMD
is the change in one sinusoidal input caused by the presence of
another sinusoidal input at a different frequency.
If two pure sine waves of frequencies fa and fb are applied
to the ADC input, nonlinearities in the ADC transfer function
can create distortion products at the sum and difference
frequencies of mfa nfb, where m and n = 0, 1, 2, 3, etc. For
example, the 2nd order IMD terms include (fa + fb). If the two
input sine waves are equal in magnitude, the value (in decibels)
of the 2nd order IMD products can be expressed by the
following formula:
Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest spectral
component excluding the input signal and DC. This value is
expressed in decibels relative to the RMS value of a full-scale
FIGURE 6: tACQ vs. Source Resistance
Input Filtering
input signal.
The noise and the distortion of the input amplifier and
other circuitry must be considered since they will add to the
AS1419 noise and distortion. The small-signal bandwidth of
the sample-and-hold circuit is 20MHz. Any noise or distortion
products that are present at the analog inputs will be summed
over this entire bandwidth. Noisy input circuitry should be
filtered prior to the analog inputs to minimize noise. A simple
1-pole RC filter is sufficient for many applications. For example,
Full-Power and Full-Linear Bandwidth
The full-power bandwidth is that input frequency at which
the amplitude of the reconstructed fundamental is reduced by
3dB for a full-scale input signal.
The full-linear bandwidth is the input frequency at which
the S/(N + D) has dropped to 77dB (12.5 effective bits). The
AS1419 has been designed to optimize input bandwidth,
allowing the ADC to undersample input signals with
frequencies above the converter’s Nyquist Frequency. The
noise floor stays very low at high frequencies; S/(N + D)
becomes dominated by distortion at frequencies far beyond
Nyquist.
Figure 7 shows a 1000pF capacitor from +AIN to ground and a
Driving the Analog Input
The differential analog inputs of the AS1419 are easy to
drive. The inputs may be driven differentially or as a singleended
input (i.e., the –AIN input is grounded). The +AIN and –AIN
inputs are sampled at the same instant. Any unwanted signal
that is common mode to both inputs will be reduced by the
common mode rejection of the sample-and-hold circuit. The
inputs draw only one small current spike while charging the
sample-and-hold capacitors at the end of conversion. During
conversion, the analog inputs draw only a small leakage
current. If the source impedance of the driving circuit is low,
FIGURE 7: RC Input Filter
Austin Semiconductor, Inc. reserves the right to change products or specifications without notice.
AS1419 & AS1419A
Rev. 1.5 08/09
10
AUSTIN [ AUSTIN SEMICONDUCTOR ]
