MAX1400 Maxim, MAX1400 Datasheet - Page 23

• Manufacturer Part Number: MAX1400
• Description: %V / 18-Bit / Low-Power / Multichannel / Oversampling Sigma-Delta ADC
• Manufacturer: Maxim

The MAX1400 is optimized for ratiometric measure-
ments and includes a fully differential reference input.
Apply the reference voltage across REFIN+ and REFIN-,
ensuring that REFIN+ is more positive than REFIN-.
REFIN+ and REFIN- must be between AGND and V+.
The MAX1400 is specified with a +2.5V reference when
operating with a +5V analog supply (V+).
The MAX1400 performs analog-to-digital conversion
using a single-bit, second-order, switched-capacitor
modulator. A single comparator within the modulator
quantizes the input signal at a much higher sample rate
than the bandwidth of the signal to be converted. The
quantizer then presents a stream of 1s and 0s to the
digital filter for processing, to remove the frequency-
shaped quantization noise.
The MAX1400 modulator provides 2nd-order frequency
shaping of the quantization noise resulting from the sin-
gle bit quantizer. The modulator is fully differential for
maximum signal-to-noise ratio and minimum suscepti-
bility to power-supply noise.
The modulator operates at one of a total of eight differ-
ent sampling rates (f
frequency (f
modulator frequency control bits MF1 and MF0. Power
dissipation is optimized for each of these modes by
controlling the bias level of the modulator. Table 15
shows the input and reference sample rates.
Table 14. R
Mode—All Modulator Sampling Frequencies (MF1, MF0 = XX); X2CLK = 0; f
2.4576MHz
PGA GAIN
128
16
32
64
1
2
4
8
CLKIN
EXT
), the X2CLK bit, the CLK bit, and the
______________________________________________________________________________________
, C
M
C
EXT
) determined by the master clock
EXT
+5V, 18-Bit, Low-Power, Multichannel,
10
10
10
10
10
10
10
10
= 0pF
Values for Less than 16-Bit Gain Error in Buffered (BUFF = 1)
Reference Input
Oversampling (Sigma-Delta) ADC
C
EXT
10
10
10
10
10
10
10
10
= 50pF
Modulator
EXTERNAL RESISTANCE R
C
EXT
= 100pF
10
10
10
10
10
10
10
10
A programmable gain amplifier (PGA) with a user-
selectable gain of x1, x2, x4, x8, x16, x32, x64, or x128
(Table 6) precedes the modulator. Figure 8 shows the
default bipolar transfer function with the following illus-
trated codes: 1) PGA = 0, DAC = 0; 2) PGA = 3, DAC =
0; or 3) PGA = 3, DAC = 3.
Tables 16a and 16b show the rms noise for typical out-
put frequencies (notches) and -3dB frequencies for the
MAX1400 with f
given are for the bipolar input ranges with V
+2.50V, with no buffer (BUFF = 0) and with the buffer
inserted (BUFF = 1). These numbers are typical and
are generated at a differential analog input voltage of 0.
Figure 7 shows graphs of Effective Resolution vs. Gain
and Notch Frequency. The effective resolution values
were derived from the following equation:
The maximum possible signal divided by the noise of
the device, SNR
full-scale voltage (i.e., 2 · V
rms noise. Note that it is not calculated using peak-to-
peak output noise numbers. Peak-to-peak noise num-
bers can be up to 6.6 times the rms numbers, while
effective resolution numbers based on peak-to-peak
noise can be 2.5 bits below the effective resolution
based on rms noise, as quoted in the tables.
Effective Resolution = (SNR
C
EXT
= 500pF
10
10
10
10
10
10
10
10
EXT
dB
CLKIN
, is defined as the ratio of the input
(kΩ)
C
= 2.4576MHz. The numbers
EXT
REFIN
= 1000pF
10
10
10
10
10
10
10
10
dB
/ GAIN) to the output
- 1.76dB) / 6.02
CLKIN
Output Noise
C
EXT
= 5000pF
10
10
10
10
10
10
10
10
=
REF
PGA
23
=