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Application Note 9517
Control Amplifier to provide adequate drive for the
segmented current cells and the R2/R resistor ladder.
Reference Out (REF OUT) should be connected to the
Control Amplifier Input (CTRL AMP IN). The Control
Amplifier Output (CTRL AMP OUT) should be used to drive
the Reference Input (REF IN) and a 0.1 μ F capacitor to
analog V-(-AV EE ). This improves settling time by decoupling
switching noise from the analog output of the HI5721.
The Full Scale Output Current is controlled by the CTRL
AMP IN pin and the set resistor (R SET ). The ratio is:
I OUT (Full Scale) = (CTRL AMP IN/R SET ) x 32
The outputs I OUT and I OUT are complementary current
outputs. Current is steered to either I OUT or I OUT in
proportion to the digital input code. The sum of the two
currents is always equal to the full scale current minus one
LSB. The current output can be converted to a voltage by
using a resistor load. Both current outputs should have the
same load (50 ? typically). The output voltage is:
V OUT = I OUT x R OUT
The compliance range of the outputs is from -1.5V to +3.0V.
HI5714 Characterization
Various tests can be used to characterize the performance
of the HI5714. The integral nonlinearity (INL) and differential
nonlinearity (DNL) specs are considered a measure of the
low frequency characteristics of the ADC. These parameters
are evaluated at the factory using a histogram approach with
a low frequency ramp input.
A three bit reconstruction DAC, as shown in Figure 7, can be
constructed to do a rough evaluation of HI5714 for DNL,
missing codes, and transition noise.
1K
DOUT2
For example, if k = 10, n = 8, m = 16, F S = 20 MSPS, and
FSR = 1V then the input ramp would have a V P-P of 62.5mV
and a period (T) of 8 μ s. To view the reconstructed output,
connect the X axis of an oscilloscope to the ramp input and
the Y axis would be connected to the reconstruction DAC
output. Another oscilloscope could be used to probe the bits
to verify the codes that are being tested. The analog input
should be low pass filtered to remove as much noise as
possible. Notice that the input ramp is only covering m steps
out a possible 2 n possible for the ADC. Therefore, the
generator used for this test will have to be able to offset the
input through the range of the converter so all the codes for
the ADC can be inspected.
Figure 8 shows what an ideal reconstructed output would
look like with and without various errors. For an ideal ADC
and an ideal ramp input, the digital output code will change
state by 1 LSB every kth conversion for an 1 LSB change on
the input. ADC errors will make the codes change before or
after the kth conversion and will translate to a larger or
smaller step width. The actual step width size would be
compared with the ideal LSB size to determine errors. Since
this is a visual comparison it will tend not to be very precise.
RAMP INPUT
111
110
101
DOUT1
2K
4K
OSCILLOSCOPE
100
011
B
DOUT0
FIGURE 7. THREE BIT RECONSTRUCTION DAC
010
001
The input frequency is set so that the input will changes by
1 LSB for every k conversions of the ADC. The p-to-p voltage
of the staircase is then determined by the number of LSB
000
1 LSB
MAJOR TRANSITION
NOISE
B - MISSING
CODE
V P – P = ------------------------
m × k
F S
steps within one period of the input ramp. The following
equations can be used:
m × FSR
2 n
T = --------------
Where:
F S = sampling frequency of the ADC.
FSR = full scale range of the ADC.
k = desired test resolution (number of conversions per LSB).
m = desired number of steps (LSBs) per ramp period.
n = number of bits of the ADC.
4
FIGURE 8. THREE BIT DAC WAVEFORMS
Further dynamic testing is used to evaluate the HI5714
performance as the input starts to approach Nyquist (F S /2).
Among these tests are Signal-to-Noise Ratio (SNR), Signal-
to-Noise And Distortion (SINAD), and Total Harmonic
Distortion (THD).
Coherent testing is recommended in order to avoid the
inaccuracies due to windowing. Coherent sampling is
governed by the following relationship: F T /F S = M/N. Where
F T is the frequency of the input tone, F S is the sampling
frequency, N is the number of samples, and M is the number
of cycles over which the samples are taken. By making M an
integer and prime (1, 3, 5. . .) the samples are assured of
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