Table 13c. r, 4576mhz – Rainbow Electronics MAX1401 User Manual
Page 24
MAX1401
+3V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
24
______________________________________________________________________________________
Input Buffers
The MAX1401 provides a pair of input buffers to isolate
the inputs from the capacitive load presented by the
PGA/modulator (Figure 6). The buffers are chopper sta-
bilized to reduce the effect of their DC offsets and low-
frequency noise. Since the buffers can represent more
than 50% of the total analog power dissipation, they
may be shut down in applications where minimum
power dissipation is required and the capacitive input
load is not a concern. Disable the buffers in applications
where the inputs must operate close to AGND or V+.
When used in buffered mode, the buffers isolate the
inputs from the sampling capacitors. The sampling-
related gain error is dramatically reduced in this mode.
A small dynamic load remains from the chopper stabi-
lization. The multiplexer exhibits a small input leakage
current of up to 10nA. With high source resistances,
this leakage current may result in a DC offset.
Table 13c. R
EXT
, C
EXT
Values for Less than 16-Bit Gain Error in Unbuffered (BUFF = 0)
Mode; 4x Modulator Sampling Frequency (MF1, MF0 = 10 ); X2CLK = 0; f
CLKIN
=
2.4576MHz
Table 13d. R
EXT
, C
EXT
Values for Less than 16-Bit Gain Error in Unbuffered (BUFF = 0)
Mode; 8x Modulator Sampling Frequency (MF1, MF0 = 11); X2CLK = 0; f
CLKIN
=
2.4576MHz
R
EXT
C
EXT
R
MUX
C
PIN
MUXOUT
ADCIN
R
IN
C
ST
C
PIN
C
AMP
C
SAMPLE
C
C
Figure 6. Analog Input, Buffered Mode (BUFF = 1)
7.0
3.4
7.0
3.4
2.3
2
5.5
3.0
3.8
2.4
1.7
8, 16, 32,
64, 128
2.1
2.3
4
1
C
EXT
= 0pF
C
EXT
= 50pF
C
EXT
= 100pF
0.71
0.40
0.71
0.40
0.11
0.66
0.38
0.60
0.34
0.09
0.10
PGA GAIN
0.11
C
EXT
= 500pF
C
EXT
= 1000pF
C
EXT
= 5000pF
EXTERNAL RESISTANCE, R
EXT
(k
Ω
)
3.4
1.7
3.4
1.7
1.1
2
2.7
1.4
1.8
1.2
0.85
8, 16, 32,
64, 128
1.0
1.1
4
1
C
EXT
= 0pF
C
EXT
= 50pF
C
EXT
= 100pF
0.35
0.20
0.35
0.20
0.05
0.32
0.18
0.29
0.17
0.04
0.05
PGA GAIN
0.05
C
EXT
= 500pF
C
EXT
= 1000pF
C
EXT
= 5000pF
EXTERNAL RESISTANCE, R
EXT
(k
Ω
)