MAX113 Datasheet, PDF (10/12 Pages) Maxim Integrated Products – +3V, 400ksps, 4/8-Channel, 8-Bit ADCs with 1UA Power-Down

+3V, 400ksps, 4/8-Channel,
8-Bit ADCs with 1µA Power-Down
+3V
MAX872
0.1µF
C1
4.7µF
V_DD
MAX113
REF+ MAX117
PWRDN
N-FET*
REF-
0.1µF
* IRML2402
PWRDN
MUX
V_IN2
R_IN
...
MAX113
MAX117
R_ON
T/H
Figure 7d. An N-channel MOSFET switches off the reference
load during power-down
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 1.5kΩ 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.
Figure 8. Equivalent Input Circuit
V_{IN_}
R _{1 VIN}
2k
22pF
10pF
MAX113
MAX117
Figure 9. RC Network Equivalent Input Model
OUTPUT CODE
11111111
11111110
11111101
FULL-SCALE
TRANSITION
1LSB = ^{VREF+ - VREF-}
256
00000011
00000010
00000001
00000000
V_REF-
123
INPUT VOLTAGE (LSBs)
Figure 10. Transfer Function
FS
FS - 1LSB
V_REF+
10 ______________________________________________________________________________________