Rainbow Electronics MAX1249 - Manual (Page 17)

MAX1248/MAX1249
+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
Lowest Power at Higher Throughputs
Figure 13b shows the power consumption with
external-reference compensation in fast power-down,
with one and four channels converted. The external
4.7µF compensation requires a 75µs wait after power-up
with one dummy conversion. This circuit combines fast
multi-channel conversion with lowest power consumption
possible. Full power-down mode may provide increased
power savings in applications where the
MAX1248/MAX1249 are inactive for long periods of time,
but where intermittent bursts of high-speed conversions
are required.
Internal and External References
The MAX1248 can be used with an internal or external
reference voltage, whereas an external reference is
required for the MAX1249. An external reference can
be connected directly at VREF or at the REFADJ pin.
An internal buffer is designed to provide 2.5V at VREF
for both the MAX1248 and the MAX1249. The
MAX1248’s internally trimmed 1.21V reference is
buffered with a gain of 2.06. The MAX1249’s REFADJ
pin is also buffered with a gain of 2.06 to scale an
external 1.25V reference at REFADJ to 2.5V at VREF.
Internal Reference (MAX1248)
The MAX1248’s full-scale range with the internal refer-
ence is 2.5V with unipolar inputs and ±1.25V with bipo-
lar inputs. The internal-reference voltage is adjustable
to ±1.5% with the circuit of Figure 15.
1
10
100
0.01 0.1 1 10 100 1k
CONVERSION RATE (Hz)
AVERAGE SUPPLY CURRENT (
µ
A)
1 CHANNEL
R LOAD = ∞
CODE = 1010101000
4 CHANNELS
Figure 13a. MAX1248 Supply Current vs. Conversion Rate,
FULLPD
1
10
100
10,000
1000
0.1 10 1 100 1k 10k 100k 1M
CONVERSION RATE (Hz)
AVERAGE SUPPLY CURRENT (
µ
A)
R
LOAD
= ∞
CODE = 1010101000
4 CHANNELS
1 CHANNEL
3.0
2.5
2.0
1.5
1.0
0.5
0
0 0.01 0.1 1 10
TIME IN SHUTDOWN (sec)
POWER-UP DELAY (ms)
Figure 13b. MAX1248 Supply Current vs. Conversion Rate,
FASTPD
Figure 13c. Typical Reference-Buffer Power-Up Delay vs. Time
in Shutdown
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