Initial power-up, Bypassing, Analog inputs – Rainbow Electronics MAX117 User Manual
Page 10
MAX113/MAX117
Initial Power-Up
When power is first applied, perform a conversion to
initialize the MAX113/MAX117. Disregard the output
data.
Bypassing
Use a 4.7µF electrolytic in parallel with a 0.1µF ceramic
capacitor to bypass V
DD
to GND. Minimize capacitor
lead lengths.
Bypass the reference inputs with 0.1µF capacitors, as
shown in Figures 7a, 7b, and 7c.
Analog Inputs
Figure 8 shows the equivalent circuit of the MAX113/
MAX117 input. When a conversion starts and
WR is
low, V
IN_
is connected to sixteen 0.6pF capacitors.
During this acquisition phase, the input capacitors
charge to the input voltage through the resistance of
the internal analog switches. In addition, about 22pF of
stray capacitance must be charged. The input can be
modeled as an equivalent RC network (Figure 9). As
source impedance increases, the capacitors take
longer to charge.
The typical 32pF input capacitance allows source resis-
tance as high as 1.5k
Ω
without setup problems. For
larger resistances, the acquisition time (t
ACQ
) must be
increased.
Internal protection diodes, which clamp the analog
input to V
DD
and GND, allow the channel input pins to
swing from GND - 0.3V to V
DD
+ 0.3V without damage.
However, for accurate conversions near full scale and
zero scale the inputs must not exceed V
DD
by more
than 50mV or be lower than GND by 50mV.
+3V, 400ksps, 4/8-Channel,
8-Bit ADCs with 1µA Power-Down
10
______________________________________________________________________________________
R
ON
R
IN
V
IN2
MAX113
MAX117
.
.
.
T/H
MUX
2k
R
V
IN_
1
22pF
V
IN
MAX113
MAX117
10pF
Figure 8. Equivalent Input Circuit
Figure 9. RC Network Equivalent Input Model
REF-
MAX113
MAX117
V
DD
MAX872
REF+
+3V
0.1µF
C1
4.7µF
PWRDN
PWRDN
N-FET*
* IRML2402
0.1µF
Figure 7d. An N-channel MOSFET switches off the reference
load during power-down
OUTPUT CODE
INPUT VOLTAGE (LSBs)
FS
FS - 1LSB
FULL-SCALE
TRANSITION
1
2
3
11111111
11111110
11111101
00000011
00000010
00000001
00000000
1LSB =
V
REF+
- V
REF-
256
V
REF-
V
REF+
Figure 10. Transfer Function