MX674A MAXIM [Maxim Integrated Products], MX674A Datasheet - Page 12

• Manufacturer Part Number: MX674A
• Description: Industry-Standard, Complete 12-Bit ADCs
• Manufacturer: MAXIM [Maxim Integrated Products]

The MAX174/MX574A/MX674A power supplies must be
filtered, well regulated, and free from high-frequency
noise, or unstable output codes will result. Unless great
care is taken in filtering any switching spikes present in
the output, switching power supplies is not suggested for
applications requiring 12-bit resolution. Take note that a
few millivolts of noise converts to several error counts in
a 12-bit ADC.
All power-supply pins should use supply decoupling capac-
itors connected with short lead length to the pins, as shown
in
directly to AGND. A 4.7µF tantalum type in parallel with a
0 1µF disc ceramic type is a suitable decoupling.
The MAX174/MX574A/MX674A have an internal buried
zener reference that provides a 10V, low-noise and low-
temperature drift output. An external reference voltage
can also be used for the ADC. When using ±15V sup-
plies, the internal reference can source up to 2mA in
addition to the BIPOFF and REFIN inputs over the entire
operating temperature range. With ±12V supplies, the
reference can drive the BIPOFF and REFIN inputs over
temperature, but it CANNOT drive an additional load.
The input leads to AGND and 10V
as short as possible to minimize noise pick up. If long
leads are needed, use shielded cables.
Figure 9. Power-Supply Bypassing
Figure
C
C
1
4
+12V/15V
-12V/15V
GROUND
GROUND
, C
, C
ANALOG
DIGITAL
2
5
, C
, C
+5V
4
6
9. The V
– 0.1µF CERAMIC
– 4.7µF
RECOMMENDED
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CC
C
C
C
4
5
6
and V
Industry-Standard, Complete 12-Bit ADCs
Power-Supply Bypassing
C
C
C
1
2
3
EE
pins should be decoupled
Driving the Analog Input
Internal Reference
IN
V
DGND
V
AGND
V
L
CC
EE
or 20V
MAX174
MX574A
MX674A
IN
should be
MAX174/MX574A/MX674A
When using the 20V
tance on the 10V
the faster MAX174, leave the 10V
capacitance and to prevent linearity errors caused by
inadequate settling time.
The amplifier driving the analog input must have low
enough DC output impedance for low full-scale error.
Furthermore, low AC output impedance is also required
since the analog input current is modulated at the clock
rate during the conversion. The output impedance of an
amplifier is the open-loop output impedance divided by
the loop gain at the frequency of interest.
MX574A and MX674A—The approximate internal clock
rate is 600kHz and 1MHz, respectively, and amplifiers
like the MAX400 can be used to drive the input.
MAX174—The internal clock rate is 2MHz and faster
amplifiers like the OP-27, AD711, or OP-42 are required.
The analog input to the ADC must be stable to within
1/2 LSB during the entire conversion for specified 12-bit
accuracy. This limits the input signal bandwidth to a
couple of hertz for sinusoidal inputs even with the faster
MAX174. For higher bandwidth signals, a track-and-hold
amplifier should be used.
The STS output may be used to provide the Hold signal
to the track-and-hold amplifier. However, since the A/D’s
DAC is switched at approximately the same time as the
conversion is initiated, the switching transients at the out-
put of the T/H caused by the DAC switching may result in
code dependent errors. It is recommended that the Hold
signal to the T/H amplifier precede a conversion or be
coincident with the conversion start.
The first bit decision by the A/D is made approximately
1.5 clock cycles after the start of the conversion. This is
2.5µs, 1.5µs, and 0.8µs for the MX574A, MX674A, and
MAX174, respectively. The T/H hold settling time must
be less than this time. For the MX574A and MX674A, the
AD585 sample-and-hold is recommended
For the MAX174, a faster T/H amplifier, like the HA5320
or HA5330, should be used
The MAX174/MX574A/MX674A input range can be set
using pin strapping. Table 3 shows the possible input
ranges and ideal transition voltages. End-point errors can
be adjusted in all ranges.
IN
pin must be minimized. Especially on
IN
as the analog input, load capaci-
Track-and-Hold Interface
(Figure
Input Configurations
IN
pin open to minimize
11).
(Figure
10).