MAX1084 Maxim, MAX1084 Datasheet - Page 9

• Manufacturer Part Number: MAX1084
• Description: 400ksps/300ksps / Single-Supply / Low-Power / Serial 10-Bit ADCs with Internal Reference
• Manufacturer: Maxim

DAC, which in turn forms a digital representation of the
analog input signal. At the conversion’s end, the input
side of C
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
the signal and the minimum time needed for the signal
to be acquired. Acquisition time is calculated by:
where R
impedance, and t
(MAX1284) or 625ns (MAX1085). Source impedance
below 4kΩ does not significantly affect the ADC’s AC
performance.
Higher source impedances can be used if a 0.01µF
capacitor is connected to the analog input. Note that
the input capacitor forms an RC filter with the input
source impedance, limiting the ADC’s input signal
bandwidth.
The ADC’s input tracking circuitry has a 6MHz
(MAX1084) or 3MHz (MAX1085) small-signal band-
width, so it is possible to digitize high-speed transient
events and measure periodic signals with bandwidths
exceeding the ADC’s sampling rate by using under-
sampling techniques. To avoid aliasing of unwanted
high-frequency signals into the frequency band of inter-
est, anti-alias filtering is recommended.
Internal protection diodes, which clamp the analog
input to V
GND - 0.3V to V
If the analog input exceeds 50mV beyond the supplies,
limit the input current to 2mA.
The MAX1084/MAX1085 have an on-chip voltage refer-
ence trimmed to 2.5V. The internal reference output is
connected to REF and also drives the internal capacitive
DAC. The output can be used as a reference voltage
source for other components and can source up to
800µA. Bypass REF with a 4.7µF capacitor. Larger
capacitors increase wake-up time when exiting shut-
down (see Using SHDN to Reduce Supply Current). The
internal reference is disabled in shutdown (SHDN = 0).
ACQ
, is the maximum time the device takes to acquire
400ksps/300ksps, Single-Supply, Low-Power,
IN
DD
HOLD
= 800Ω, R
Serial 10-Bit ADCs with Internal Reference
and GND, allow the input to swing from
t
ACQ
DD
switches back to AIN, and C
_______________________________________________________________________________________
+ 0.3V without damage.
= 7(R
ACQ
S
S
= the input signal’s source
is never less than 468ns
+ R
Analog Input Protection
IN
)
✕
12pF
Internal Reference
Input Bandwidth
HOLD
When power is first applied, and if SHDN is not pulled
low, it takes the fully discharged 4.7µF reference
bypass capacitor up to 1.4ms to acquire adequate
charge for specified accuracy. No conversions should
be performed during this time.
Figure 3. Typical Operating Circuit
Figure 4. Equivalent Input Circuit
ANALOG INPUT
SHUTDOWN
AIN
*INCLUDES ALL INPUT PARASITICS
+3V to +5V
0 TO V
INPUT
REF
10µF
C
GND
SWITCH
REF
6pF
Initialization After Power-Up and
0.1µF
*
4.7µF
CAPACITIVE DAC
C
HOLD
1
2
3
4
12pF
HOLD
V
AIN
SHDN
REF
DD
MAX1084
MAX1085
Starting a Conversion
TRACK
R
800Ω
IN
Serial Interface
ZERO
DOUT
SCLK
GND
CS
8
7
6
5
AUTOZERO
RAIL
COMPARATOR
SERIAL
INTERFACE
9