MAX1365 Datasheet, PDF (12/25 Pages) Maxim Integrated Products – Stand-Alone, 4.5-/3.5-Digit Panel Meters with 4-20mA Output

Stand-Alone, 4.5-/3.5-Digit Panel Meters
with 4–20mA Output
signal into a digital pulse train whose average duty
cycle represents the digitized signal information. The
modulator quantizes the input signal at a much higher
sample rate than the bandwidth of the input. The
MAX1365/MAX1367 modulator provides 3rd-order fre-
quency shaping of the quantization noise resulting from
the single-bit quantizer. The modulator is fully differen-
tial for maximum signal-to-noise ratio and minimum sus-
ceptibility to power-supply noise. A single-bit data
stream is then presented to the digital filter to remove
the frequency-shaped quantization noise.
Digital Filtering
The MAX1365/MAX1367 contain an on-chip digital low-
pass filter that processes the data stream from the
modulator using a SINC⁴ response:
 sin(x) ⁴
 x 
The SINC⁴ filter has a settling time of four output data
periods (4 x 200ms). The MAX1365/MAX1367 have
25% overrange capability built into the modulator and
digital filter. The digital filter is optimized for the f_CLK
equal to 4.9152MHz. The frequency response of the
SINC⁴ filter is calculated as follows:
H(z)
=
_1(1−
Z ^−N )
⁴

 ^{N (1−Z−1)} 
H(f ) =
1
N




sin_Nπ
f
f_m




sin_
πf
f_m




⁴





where N is the oversampling ratio, and f_m = N x output
data rate = 5Hz.
Filter Characteristics
Figure 1 shows the filter frequency response. The
SINC⁴ characteristic -3dB cutoff frequency is 0.228
times the first notch frequency (5Hz). The oversampling
ratio (OSR) for the MAX1367 is 128 and the OSR for the
MAX1365 is 1024. The output data rate for the digital fil-
ter corresponds to the positioning of the first notch of
the filter’s frequency response. The notches of the
SINC⁴ filter are repeated at multiples of the first notch
frequency. The SINC⁴ filter provides an attenuation of
better than 100dB at these notches. For example, 50Hz
is equal to 10 times the first notch frequency and 60Hz
is equal to 12 times the first notch frequency. For large
0
-40
-80
-120
-160
-200
0
10 20 30 40 50 60
FREQUENCY (Hz)
Figure 1. Frequency Response of the SINC⁴ Filter (Notch at 60Hz)
step changes at the input, allow a settling time of
800ms before valid data is read.
Internal Clock
The MAX1365/MAX1367 contain an internal oscillator.
Using the internal oscillator saves board space by
removing the need for an external clock source. The
oscillator is optimized to give 50Hz and 60Hz power-
supply and common-mode rejection.
Charge Pump
The MAX1365/MAX1367 contain an internal charge pump
to provide the negative supply voltage for the internal
analog input/reference buffers. The bipolar input range of
the analog input/reference buffers allows this device to
accept negative inputs with high source impedances.
Connect a 0.1µF capacitor from NEGV to GND.
LED Driver (Table 1)
The MAX1365 has a 4.5-digit common-cathode display
driver, and the MAX1367 has a 3.5-digit common-cath-
ode display driver. In addition, the LED drivers of the
MAX1365/MAX1367 feature peak-detection and data-
hold circuitry.
Figures 2 and 3 show the connection schemes for a
standard seven-segment LED display. The LED update
rate is 2.5Hz. Figure 4 shows a typical common-cath-
ode configuration for two digits. In common-cathode
configuration, the cathodes of all LEDs in a digit are
connected together. Each segment driver of the
MAX1365/MAX1367 connects to its corresponding
LED’s anodes. For example, segment driver SEGA con-
nects to all LED segments designated as A. Similar
configurations are used for other segment drivers.
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