Table 13c. r, 4576mhz – Rainbow Electronics MAX1400 User Manual
Page 22
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
22
______________________________________________________________________________________
Input Buffers
The MAX1400 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)
9.3
4.5
9.3
4.5
3.0
2
7.2
3.9
5.0
3.1
2.3
8, 16, 32,
64, 128
2.7
3.0
4
1
C
EXT
= 0pF
C
EXT
= 50pF
C
EXT
= 100pF
0.94
0.53
0.94
0.53
0.14
0.87
0.50
0.79
0.45
0.12
0.13
PGA GAIN
0.14
C
EXT
= 500pF
C
EXT
= 1000pF
C
EXT
= 5000pF
EXTERNAL RESISTANCE R
EXT
(k
Ω
)
4.5
2.2
4.5
2.2
1.5
2
3.5
1.9
2.4
1.5
1.1
8, 16, 32,
64, 128
1.3
1.5
4
1
C
EXT
= 0pF
C
EXT
= 50pF
C
EXT
= 100pF
0.46
0.26
0.46
0.26
0.069
0.42
0.24
0.38
0.22
0.059
0.069
PGA GAIN
0.069
C
EXT
= 500pF
C
EXT
= 1000pF
C
EXT
= 5000pF
EXTERNAL RESISTANCE R
EXT
(k
Ω
)