Datasheet

R <
SOURCE
t
ACQ
C ln(2)(n+1)
S
- (R +R )
SER SW
ADS8528
ADS8548
ADS8568
SBAS543A AUGUST 2011 REVISED OCTOBER 2011
www.ti.com
A driving operational amplifier may not be required, if the impedance of the signal source (R
SOURCE
) fulfills the
requirement of Equation 2:
(2)
where:
n = 12, 14, or 16; n is the resolution of the ADC,
C
S
= 10pF is the sample capacitor value in V
IN
= ±4VREF mode,
R
SER
= 200Ω is the input resistor value,
and R
SW
= 130Ω is the switch resistance value.
With a minimum acquisition time of t
ACQ
= 280ns, the maximum source impedance should be less than 2.7kΩ for
the ADS8528, 2.3kΩ for the ADS8548, and 2.0kΩ for the ADS8568 in ±4VREF mode, or less than 1.2kΩ for the
ADS8528, 1.0kΩ for the ADS8548, and 0.8kΩ for the ADS8568 in ±2VREF mode. The source impedance can be
higher if the application allows longer acquisition time.
Analog-to-Digital Converter (ADC)
The device includes eight ADCs that operate with either an internal or an external conversion clock.
Conversion Clock
The device uses either an internally-generated (CCLK) or an external (XCLK) conversion clock signal (in
software mode only). In default mode, the device generates an internal clock. In this case, a complete conversion
including the pre-charging of the sample capacitors takes 19 to 20 clock cycles, depending on the setup time of
the incoming CONVST_x signal with relation to the rising edge of the CCLK.
When CLKSEL bit is set high (CONFIG bit C29), an external conversion clock can be applied on pin 34. A
complete conversion process requires 19 clock cycles in this case if the t
SCVX
timing requirement is fulfilled. The
external clock can remain low between conversions.
If the application requires lowest power dissipation at low data rates, it is recommended to use the auto-sleep
mode, activated using pin 36 (ASLEEP). In this case, a conversion cycle takes up to 26 clock cycles (see the
Reset and Power-Down Modes section for more details).
26 Copyright © 2011, Texas Instruments Incorporated
Product Folder Link(s): ADS8528 ADS8548 ADS8568