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> A/D and
D/A Conversion/Sampling Circuits > APP
4442
Maxim > Design support >
App notes
> Display
Drivers > APP 4442

Sep 21, 2009

Keywords: logarithmic panel displays, LED panels,
analog-digital converters, ADCs
Related
parts

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APPLICATION NOTE 4442

Logarithmic LED Panel Display Has 72dB Range

By: Tamer
Mogannam

Abstract: This
application note presents a simple, stand-alone panel-display
circuit that drives a 12-LED logarithmic column. The LED
column is a 12-step log indicator with 6dB step height,
forming a total column range of 72dB. The circuit's accuracy
and stability are defined by the specifications for the
design's ADC. The MAX1276 12-bit ADC is featured.

This
design idea appeared in the October 31, 2006 issue of EE
Times magazine.

Panel-level logarithmic-column
displays are usually built in one of two configurations: an analog
logarithmic converter and a column-display ADC; or the
combination of a high-resolution ADC,
a processor-controlled logarithmic algorithm, and an LED or LCD
column driver.

There is a simpler, all-digital
alternative consisting of a serial-output ADC and some timing
and display electronics (Figure 1). A stand-alone
circuit, it requires no trimming (as analog versions do), no
microcontroller, and no software. The signal to be displayed
is applied to a 12-bit ADC (IC1, the MAX1276). For the
circuit shown, that signal can range from zero (no LED on) to
+2.048V (all LEDs on), with the first LED coming on at
+1mV.

When triggered by a short positive pulse at
CNVST, the MAX1276 begins a conversion, clocked by the signal
at SCLK. Its output (DOUT) is clocked by the rising edge of
SCLK and starts with four leading zeros, followed by the
12-bit conversion result, MSB
first. Thus, one conversion result requires 16 clock pulses at
SCLK.

The display is a vertical stack of 12 LEDs in
which the top LED represents the MSB. During operation, the
circuit scans each conversion result as it is generated (MSB
first, as described above). It notes the first bit with a
value of "1," and then proceeds to illuminate that LED and all
those below it. The result is a logarithmic column in which
the input-voltage change necessary to move the column one step
up or down is double or half the current
input value (a 6.02dB step). The number of steps available
equals the ADC's resolution (12 bits in this
case).

Figure
1. This stand-alone panel-display circuit drives a 12-LED
logarithmic column, whose height changes by one LED for each
6.02dB change in the input voltage.

At DOUT, the
first output bit with a value of "1" charges C1 (with diode
D1) to the logic-one level. The voltage on C1 connects to the
data input (DS) of the first of two cascaded 74HC595 ICs,
which together form a 16-bit shift
register. The signal that clocks the ADC, slightly
delayed, also clocks the shift register, thereby inserting
into the shift register the value presented at its input. At
the end of the conversion, all bits following the first one to
exhibit a "1" value are also forced to "1" by the voltage
stored on C1.

After each conversion is finished, a
negative pulse is applied to the SC_TP inputs of both 74HC595
ICs (Figure 2). This action transfers the internal
shift-register contents to an internal parallel-output
register. The same pulse discharges the storage capacitor
through diode D2, leaving the circuit ready for the next
conversion scan. The parallel-register outputs then serve as
the column LED drivers. A 74HC4060 IC serves as clock and
timing sequence generator, and a 74HC132 provides some
necessary glue logic.

Figure
2. Timing for the Figure 1 circuit. CNVST: MAX1276 conversion
trigger signal; SCLK: MAX1276 conversion clock, and 74HC595
shift clock; DOUT: MAX1276 conversion data out; DS: 74HC595
shift data input; ST_CP: 74HC595 shift-register to
parallel-register transfer clock pulse.

Thus, the
LED column is a 12-step log indicator with 6dB step height,
forming a total column range of 72dB. Its accuracy and
stability are those defined by the specifications for the
MAX1276. The sampling
rate for the display is about 2.5kHz with the component
values shown.

Related parts

MAX1276

1.8Msps, Single-Supply,
Low-Power, True-Differential, 12-Bit ADCs with
Internal Reference

Free
samples

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Download,
PDF format (68kB)
APP 4442: Sep 21, 2009

AN4442, AN 4442, APP4442,
Appnote4442, Appnote 4442

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