MX7828KP Maxim Integrated Products, MX7828KP Datasheet - Page 9

• Manufacturer Part Number: MX7828KP
• Description: ADC (A/D Converters) 8-Bit 8Ch Precision ADC
• Manufacturer: Maxim Integrated Products

Specifications of MX7828KP

• Architecture: Semiflash
• Conversion Rate: 400 KSPs
• Input Type: Voltage
• Maximum Power Dissipation: 75 mW
• Maximum Operating Temperature: + 70 C
• Minimum Operating Temperature: 0 C
• Lead Free Status / Rohs Status: No

Figure 8a. Internal Reference (MAX154/MAX158 only)
Figure 8b. External Reference +2.5V Full-Scale
Figure 8c. Power Supply as Reference
Figure 8d. Inputs Not Referenced to GND
+5V
+5V
+5V
0.1 F
0.1 F
0.1 F
+5V
* Current path must
still exist from
V
IN(-)
0.1 F
to Ground
47 F
47 F
47 F
47 F
AIN
AIN
AIN
AIN
_______________________________________________________________________________________
AIN
AIN
AIN
AIN
x
x
x
x
(+)
x
x
(-)
(+)
(+)
(-)
(-)
x
x
(+)
(-)
MX584
0.01 F
2.5V
*
2.5V
V
GND
V
V
V
GND
V
REF OUT
V
V
GND
V
V
V
V
V
V
GND
V
V
V
IN
DD
REF
IN
DD
REF
REF
REF
IN
DD
REF
IN
DD
REF
REF
REF
+
+
-
-
+
-
+
-
CMOS, High-Speed, 8-Bit ADCs
MAX154
MAX158
MAX154
MAX158
MAX154
MAX158
MX7824
MX7828
MX7824
MX7828
MX7824
MX7828
The converters’ analog inputs behave somewhat differ-
ently from conventional ADCs. The sampled data com-
parators take varying amounts of current from the input,
depending on the cycle they are in. The equivalent cir-
cuit of the converter is shown in Figure 9a. When the
conversion starts, AIN(n) is connected to the MS and
LS comparators. Thus, AIN(n) is connected to thirty-one
1pF capacitors.
To acquire the input signal in approximately 1µs, the
input capacitors must charge to the input voltage
through the on-resistance of the multiplexer (about
600Ω) and the comparator’s analog switches (2kΩ to
5kΩ per comparator). In addition, about 12pF of stray
capacitance must be charged. The input can be mod-
eled as an equivalent RC network shown in Figure 9b.
As R
take longer to charge.
Since the length of the input acquisition time is internal-
ly set, large source resistances (greater than 100Ω) will
cause settling errors. The output impedance of an op-
amp is its open-loop output impedance divided by the
loop gain at the frequency of interest. It is important
that the amplifier driving the converter input have suffi-
cient loop gain at approximately 1MHz to maintain low
output impedance.
The transients in the analog input caused by the sam-
pled data comparators do not degrade the converter’s
performance, since the ADC does not “look” at the
input when these transients occur. The comparator’s
outputs track the input during the first 1µs of the con-
version, and are then latched. Therefore, at least 1µs
will be provided to charge the ADC’s input capaci-
tance. It is not necessary to filter these transients with
an external capacitor on the AIN terminals.
The MAX154/MAX158 and MX7824/MX7828 can mea-
sure input signals with slew rates as high as 157mV/µs
to the rated specifications. This means that the analog
input frequency can be as high as 10kHz without the
aid of an external track/hold. The maximum sampling
rate is limited by the conversion time (typical t
2µs) plus the time required between conversions (t
500ns). It is calculated as:
f
MAX
S
(source impedance) increases, the capacitors
=
t
CRD
with Multiplexer
1
+ t
p
=
(2.0 + 0.5) µs
1
Sinusoidal Inputs
Input Filtering
Input Current
= 400kHz
CRD
p
=
=
9