ZADCS146_11 Datasheet, PDF (17/26 Pages) List of Unclassifed Manufacturers – 12-Bit, 200ksps, ADC Family

ZADCS146/147
12-Bit, 200ksps, ADC Family
The small-signal bandwidth of the input tracking circuitry is 3.8MHz. Hence it is possible to digitize high-speed
transient events and periodic signals with frequencies exceeding the ADC’s sampling rate. This allows the
application of certain under-sampling techniques like down conversion of modulated high frequency signals.
Be aware that under-sampling techniques still require a bandwidth limitation of the input signal to less than the
Nyquist frequency of the converter to avoid aliasing effects. Also, the output impedance of the input source
must be very low to achieve the mentioned small signal bandwidth in the overall system.
2.3. Internal & External Reference
ZADCS146 is equipped with a highly accurate internal 2.5V reference voltage source. The voltage is
generated from a trimmed 1.25V bandgap with an internal buffer that is set to a gain of 2.00. The bandgap
voltage is supplied at VREFADJ with an output impedance of 20kΩ. An external capacitor of 47nF at
VREFADJ is useful to further decrease noise on the internal reference.
The VREFADJ pin also provides an opportunity to externally adjust the bandgap voltage in a limited range
(see Figure 8) as well as the possibility to overdrive the internal bandgap with an external 1.25V reference.
The internal bandgap reference and the VREF buffer can be shut down completely by setting VREFADJ to
VDD. This reduces power consumption of the ZADCS146 device and allows the supply of an external
reference at VREF.
VDD = +2.7V … +5.25V
510kΩ
47nF
ZADCS146
VREFADJ
ZADCS147 does not contain the internal bandgap or the VREF
buffer. An external reference must be supplied all the time at
VREF.
The value of the reference voltage at VREF sets the input range
of the converter and the analog voltage weight of each digital
code. The size of the LSB (least significant bit) is equal to the
value of VREF (reference to AGND) divided by 4096. For
example at a reference voltage of 2.500V, the voltage level of a
LSB is equal to 0.610mV.
The average current consumption at VREF depends on the value
Figure 8: Reference Adjust Circuit
of VREF and the sampling frequency. Two effects contribute to
the current at VREF, a resistive connection from VREF to AGND
and charge currents that result from the switching and recharging of the capacitor array (CDAC) during
sampling and conversion. For an external reference of 2.5V the input current at VREF is approximately
100µA.
2.4. Digital Interface
ZADCS146 and ZADCS147 are controlled by a 4-wire serial interface that is compatible to SPI™, QSPI™ and
MICROWIRE™ devices without external logic. Any conversion is started by sending a control byte into DIN
while nCS is low. A typical sequence is shown in Figure 9.
The control byte defines the input channel(s), unipolar or bipolar operation and output coding, single-ended or
differential input configuration, external or internal conversion clock and the kind of power down that is
activated after the completion of a conversion. A detailed description of the control bits can be obtained from
Figure 9. As it can also be seen in Figure 9 the acquisition of the input signal occurs at the end of the control
byte for 2.5 clock cycles. Outside this range, the Track & Hold is in hold mode.
The conversion process is started, with the falling clock edge (SCLK) of the eighth bit in the control byte. It
takes twelve clock cycles to complete the conversion and one additional cycle to shift out the last bit of the
conversion result. During the remaining seven clock cycles the output is filled with zeros in 24-Clock
Conversion Mode. Depending on what clock mode was selected, either the external SPI clock or an internal
clock is used to drive the successive approximation. Figure 10 shows the Timing for Internal Clock Mode.
Data Sheet
October 12, 2011
© 2011 Zentrum Mikroelektronik Dresden AG — Rev. 2.0
All rights reserved. The material contained herein may not be reproduced, adapted, merged, translated, stored, or used without
the prior written consent of the copyright owner. The information furnished in this publication is PRELIMINARY and subject to
changes without notice.
17 of 25