ADC
AS1419
AS1419A
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, theADC transfer function nonlinearity can produce
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 ap-
plied to the ADC input, nonlinearities in the ADC transfer
function can create distortion products at the sum and differ-
ence
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 theAS1419
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, Figure
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 band-
7 shows a 1000pF capacitor from +AIN to ground and a 100Ω
width,
with
allowing the ADC to undersample input signals
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.
Driving the Analog Input
The differential analog inputs of the AS1419 are easy to
drive. The inputs may be driven differentially or as a singleen-
ded 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,
FIGURE 7: RC Input Filter
the analog inputs draw only a small leakage
current. If the
source impedance of the driving circuit is low, then theAS1419
Micross Components reserves the right to change products or specifications without notice.
AS1419 & AS1419A
Rev. 1.7 06/10
10
MICROSS [ MICROSS COMPONENTS ]
