|ML2003CX Datasheet PDF : 11 Pages |
The ML2003 consists of a coarse gain stage, a ﬁne gain
stage, an output buffer, and a serial/parallel digital interface.
The analog input, VIN, goes directly into the op amp input in
the coarse gain stage. The coarse gain stage has a gain range
of 0 to 22.5dB in 1.5dB steps.
The ﬁne gain stage is cascaded onto the coarse section. The
ﬁne gain stage has a gain range of 0 to 1.5dB in 0.1dB steps.
In addition, both sections can be programmed for either gain
or attenuation, thus doubling the effective gain range.
The logarithmic steps in each gain stage are generated by
placing the input signal across a resistor string of 16 series
resistors. Analog switches allow the voltage to be tapped
from the resistor string at 16 points. The resistors are sized
such that each output voltage is at the proper logarithimic
ratio relative to the input signal at the top of the string. Atten-
uation is implemented by using the resistor string as a simple
voltage divider, and gain is implemented by using the resis-
tor string as a feedback resistor around an internal op amp.
Since the coarse and ﬁne gain stages are cascaded, their gains
can be summed logarithmically. Thus, any gain from –24dB to
+24dB in 0.1dB steps can be obtained by combining the
coarse and ﬁne gain settings to yield the desired gain setting.
The relationship between the digital select bits and the corre-
sponding analog gain values is shown in Tables 1 and 2. Note
that C3-C0 selects the coarse gain, F3-F0 selects the ﬁne gain,
and ATTEN/GAIN selects either attenuation or gain.
The ﬁnal analog stage is the output buffer. This ampliﬁer has
internal gain of 1 and is designed to drive 600 ohms and
100pF loads. Thus, it is suitable for driving a telephone
hybrid circuit directly without any external ampliﬁer.
The digital section is powered between VCC and GND, or 5
volts. The analog section is powered between VCC and VSS
and uses AGND as the reference point, or ±5 volts.
GND and AGND are totally isolated inside the device to
minimize coupling from the digital section into the analog
section. However, AGND and GND should be tied together
physically near the device and ideally close to the common
power supply ground connection.
Typically, the power supply rejection of VCC and VSS to the
analog output is greater than –60dB at 1 kHz. If decoupling
of the power supplies is still necessary in a system, VCC and
VSS should be decoupled with respect to AGND.
A powerdown mode can be selected with pin PDN. When
PDN = 1, the device is powered down. In this state, the power
consumption is reduced by removing power from the analog
section and forcing the analog output,VOUT, to a high
impedance state. While the device is in powerdown mode,
the digital section is still functional and the current data
word remains stored in the latch when in serial mode.
When PDN = 0, the device is in normal operation.
The ML2003 can be operated with a serial or parallel
interface. The SER/PAR pin selects the desired interface.
When SER/PAR = 1, the serial mode is selected. When
SER/PAR = 0, the parallel mode is selected. The ML2004
digital interface is serial only.
Serial mode is selected by setting SER/PAR pin high. The
serial interface allows the gain settings to be set from a serial
The timing for the serial mode is shown in Figure 10. The
serial input data, SID, is loaded into a shift register on rising
edges of the shift clock, SCK. The data can be parallel
loaded into a latch when the input latch signal, LATI, is high.
The LATI pulse must occur when SCK is low. In this way,
a new data word can be loaded into the shift register without
disturbing the existing data word in the latch.
The parallel outputs of the latch control the attenuation/gain
setting. The order of the data word bits in the latch is shown
in Figure 11. Note that bit 0 is the ﬁrst bit of the data word
clocked into the shift register. Tables 1 and 2 describe how
the data word programs the gain.
Table 1. Fine Gain Settings (C3-C0 = 0)
Ideal Gain (dB)
F3 F2 F1 F0 ATTEN/GAIN = 1 ATTEN/GAIN = 0
REV. 1.1.1 3/19/01
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