__________Applications Information
Analog Input
The main board digitizes single-ended signals by choos-
ing either input and leaving the other input either open or
terminated in the system characteristic impedance. In this
mode the unused input can provide a DC offset to the
incoming signal. (See the
Electrical Characteristics
in the
MAX101A data sheet for this DC voltage range.)
To obtain a digital output of all ones (11....1) with differen-
tial input drive for the MAX101, 270mV must be applied
between AIN+ and AIN-. That is, AIN+ = +135mV and
AIN- = -135mV (when no DC offset is applied). Mid-scale
digital output code occurs when there is no voltage differ-
ence across the analog inputs. Zero-scale digital output
code, with differential drive for the MAX101, occurs when
AIN+ = -135mV and AIN- = +135mV. The output of the
converter stays at all ones (full scale) or all zeros (zero
scale) when overranged or underranged, respectively.
Tables 1a and 1b show these relationships for both the
MAX101 and the MAX101A.
Digital Outputs
Data from the ADC is interleaved and is output on alter-
nate clock phases. One 8-bit word is output during one
clock phase and the other is output on the alternate clock
phase. The two 8-bit-wide data paths are buffered by
100E116 line receivers, which provide a differential output,
available at the connector. If the termination board is not
used, the user must provide proper ECL termination at the
EURO-card connector.
Input Reference-Resistor Strings
Operational amplifiers are used to drive the top and
bottom inputs of each of the ADC reference resistor
chains. A 2.5V reference is resistor-divided down
and buffered through two MAX412CPA op amps. (The
relatively low input impedance of each string, 120Ω,
will draw approximately 17mA.) The reference voltage
is set at the factory for either the MAX101 or MAX101A.
This reference controls the comparator input windows,
and can be adjusted between ±1.20V to accommodate
input requirements. (Accuracy specifications are
guaranteed with a reference of ±1.02V (MAX101) or
±0.95V (MAX101A).)
Testing
We recommend that a digital acquisition instrument like
the HP16500 series of logic analyzers be used to
acquire and process the output data. At Maxim, the
data acquired from the converter is evaluated in an
effective-bits software program developed in-house.
The effective-bits measurement is a good tool to deter-
mine and compare ADC accuracy. See the MAX101A
data sheet for more details on effective-bits testing.
_____________Calibration Procedure
The MAX101 EV kit comes calibrated and ready to
operate from the factory. If other MAX101A devices are
to be used in the same fixture, the EV kit should be
recalibrated according to the following procedure:
1) With the ADC removed, adjust the +5V and -5.2V
power supplies.
2) Adjust the PHASE potentiometer (R26) to a nomi-
nal voltage of 0V. A test point (TP1) for this voltage
measurement is located near the potentiometer.
3) With the power off, insert the MAX101A into the
board. The device’s heatsink fits down through the
board, and its leads rest on top of the board. Take
care to place the part in the board with Pin 1 in the
correct location. Pin 1 is indicated by a small dot
near the U1 device designation.
4) Turn the power on, observing proper sequencing,
and let the part warm up for several minutes.
Use a fan to ensure 200 lineal feet/min airflow.
Repeat Step 2.
Evaluates: MAX101/MAX101A
MAX101A Evaluation Kit
4 _______________________________________________________________________________________
*An offset V
IO
, as specified in the
DC Electrical Characteristics
,
will be present at the input. Compensate for this offset by either
adjusting the reference voltages VA
RT
, VA
RB
, VB
RT
,VB
RB
,
or introduce an offset voltage in one of the input terminals,
AIN+ or AIN-.
Table 1a. MAX101 Input Voltage Range
Table 1b. MAX101A Input Voltage Range
AIN+* AIN-*
OUTPUT
CODE
MSB TO
LSB
INPUT
Differential
+135mV
0
-135mV
-135mV
0
+135mV
11111111
10000000
00000000
full scale
mid scale
zero scale
Single
Ended
+270mV
0
-270mV
0
0
0
11111111
10000000
00000000
full scale
mid scale
zero scale
full scale
mid scale
zero scale
AIN+* AIN-*
OUTPUT
CODE
MSB TO
LSB
INPUT
Differential
+125mV
0
-125mV
-125mV
0
+125mV
11111111
10000000
00000000
full scale
mid scale
zero scale
Single
Ended
+250mV
0
-250mV
0
0
0
11111111
10000000
00000000
