ADS850 Datasheet, PDF (9/15 Pages) Texas Instruments – 14-Bit, 10MSPS Self-Calibrating ANALOG-TO-DIGITAL CONVERTER

TYPICAL CHARACTERISTICS (Cont.)
At T_A = full specified temperature range, V_S = +5V, specified input range = 1.5V to 3.5V, differential internal reference input and sampling rate = 10MSPS after calibration,
V_REF = 2V, –1dB input, unless otherwise specified.
110
100
90
80
70
60
50
40
30
–60
SWEPT POWER
dBFS
f_IN = 4.8MHz
dBc
–50
–40
–30
–20
–10
0
Input Amplitude (dBFS)
APPLICATION INFORMATION
DRIVING THE ANALOG INPUT
The ADS850 allows its analog inputs to be driven either
single-ended or differentially. The focus of the following
discussion is on the single-ended configuration.
CALIBRATION PROCEDURE
The calibration procedure (CAL) is started by a positive
pulse, with a minimum width of 2 clock cycles. Once calibra-
tion is initiated, the clock must operate continuously and the
power supplies and references must remain stable. The
calibration registers are reset on the rising edge of the CAL
signal. The actual calibration procedure begins at the falling
edge of the CAL signal. Calibration is completed at the end
of 32,775 cycles at 10MSPS, CAL = 3.28ms (see Timing
Diagram 3 on page 5). During calibration, the CAL_BUSY
signal stays HIGH and the digital output pins of the ADC are
forced to zero. Also, during calibration, the inputs (IN and IN)
are disabled. When the calibration procedure is complete,
the CAL_BUSY goes LOW. Valid data appears at the output
seven cycles later or after a total of 32,775 clock cycles. If
there are any changes to the clock or the temperature
changes more than ±20°C, the ADC should be re-calibrated
to maintain performance.
At power-on (see Timing Diagram 2 on page 5), the ADC
calibrates itself. The power-on delay, t_S, is the time it takes
for the reference voltage to settle. Once the clock starts, the
power-on delay operates for 2²¹ clock cycles. Bypass capaci-
tors should be selected to allow the reference to settle within
200ms. If the system is noisy or external references require
a longer settling time, a CAL pulse may be required.
AC-COUPLED INPUT CONFIGURATION
See Figure 1 for the circuit example of the most common
interface configuration for the ADS850. With the V_REF pin
connected to the SEL pin, the full-scale input range is defined
to be 2Vp-p. This signal is ac-coupled in single-ended form
to the ADS850 using the low distortion voltage-feedback
amplifier OPA642. As is generally necessary for single-
supply components, operating the ADS850 with a full-scale
input signal swing requires a level-shift of the amplifier’s
zero-centered analog signal to comply with the ADC’s input
range requirements. Using a DC blocking capacitor between
the output of the driving amplifier and the converter’s input,
a simple level-shifting scheme can be implemented. In this
configuration, the top and bottom references (REFT, REFB)
provide an output voltage of +3V and +2V, respectively.
Here, two resistor pairs of 2 • 2kΩ are used to create a
common-mode voltage of approximately +2.5V to bias the
inputs of the ADS850 (IN, IN) to the required DC voltage.
An advantage of ac-coupling is that the driving amplifier still
operates with a ground-based signal swing. This will keep
the distortion performance at its optimum since the signal
swing stays centered within the linear region of the op amp
and sufficient headroom to the supply rails can be main-
tained. Consider using the inverting gain configuration to
eliminate CMR induced errors of the amplifier. The addition
of a small series resistor (R_S) between the output of the op
amp and the input of the ADS850 will be beneficial in almost
all interface configurations. This will decouple the op amp’s
output from the capacitive load and avoid gain peaking,
which can result in increased noise. For best spurious and
distortion performance, the resistor value should be kept
below 100Ω. Furthermore, the series resistor together with
the 100pF capacitor establish a passive low-pass filter,
limiting the bandwidth for the wideband noise, thus help
improving the SNR performance.
ADS850
9
SBAS154C
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