Rainbow Electronics MAX1168 User Manual

Page 12

MAX1167/MAX1168

Track/Hold (T/H)

In track mode, the analog signal is acquired on the
internal hold capacitor. In hold mode, the T/H switches
open and the capacitive digital-to-analog converter
(DAC) samples the analog input.

During the acquisition, the analog input (AIN_) charges
capacitor C

DAC

. At the end of the acquisition interval

the T/H switches open. The retained charge on C

DAC

represents a sample of the input.

In hold mode, the capacitive DAC adjusts during the
remainder of the conversion cycle to restore node
ZERO to zero within the limits of 16-bit resolution. At the
end of the conversion, force CS high and then low to
reset the T/H switches back to track mode (AIN_),
where C

DAC

charges to the input signal again.

The time required for the T/H to acquire an input signal
is a function of how quickly its input capacitance is
charged. If the input signal’s source impedance is high,
the acquisition time lengthens and more time must be
allowed between conversions. The acquisition time
(t

ACQ

) is the maximum time the device takes to acquire

the signal. Use the following formula to calculate acqui-
sition time:

ACQ

= 11(R

+ R

DS(ON)

)

45pF + 0.3µs

where R

= 340

Ω, R

= the input signal’s source

impedance, R

DS(ON)

= 60

Ω, and t

ACQ

is never less

than 729ns. A source impedance of less than 200

Ω

does not significantly affect the ADC’s performance.

Multichannel, 16-Bit, 200ksps Analog-to-Digital
Converters

______________________________________________________________________________________

REFERENCE

REF

REFCAP

AGND

DGND

AIN0

AIN1

AIN2

AIN3

SCLK

DIN

ANALOG-INPUT

MULTIPLEXER

CONTROL

ACCUMULATOR

MEMORY

INPUT REGISTER

BIAS

OSCILLATOR

OUTPUT

DOUT

EOC

ANALOG-SWITCH FINE TIMING

SUCCESSIVE-APPROXIMATION

MAX1167

DAC

BUFFER

RAIL

COMPARATOR

Figure 3. MAX1167 Functional Diagram