Rainbow Electronics MAX5155 - Manual (Page 9)

_______________Detailed Description
The MAX5154/MAX5155 dual, 12-bit, voltage-output
DACs are easily configured with a 3-wire serial inter-
face. These devices include a 16-bit data-in/data-out
shift register, and each DAC has a double-buffered
input composed of an input register and a DAC register
(see
Functional Diagram ). In addition, trimmed internal
resistors produce an internal gain of +2V/V that maxi-
mizes output voltage swing. The amplifier’s offset-adjust
pin allows for a DC shift in the DAC’s output.
Both DACs use an inverted R-2R ladder network that pro-
duces a weighted voltage proportional to the input volt-
age value. Each DAC has its own reference input to
facilitate independent full-scale values. Figure 1 depicts a
simplified circuit diagram of one of the two DACs.
Reference Inputs
The reference inputs accept both AC and DC values
with a voltage range extending from 0V to (V
DD - 1.4V).
Determine the output voltage using the following equa-
tion (OS_ = AGND):
V
OUT
= (V
REF
x NB / 4096) x 2
where NB is the numeric value of the DAC’s binary input
code (0 to 4095) and V
REF
is the reference voltage.
The reference input impedance ranges from 14k
Ω
(1554
hex) to several giga ohms (with an input code of 0000
hex). The reference input capacitance is code dependent
and typically ranges from 15pF with an input code of all
zeros to 50pF with a full-scale input code.
Output Amplifier
The output amplifiers on the MAX5154/MAX5155 have
internal resistors that provide for a gain of +2V/V when
OS_ is connected to AGND. These resistors are
trimmed to minimize gain error. The output amplifiers
have a typical slew rate of 0.75V/µs and settle to
1/2LSB within 15µs, with a load of 10k
Ω
in parallel with
100pF. Loads less than 2k
Ω
degrade performance.
The OS_ pin can be used to produce an adjustable off-
set voltage at the output. For instance, to achieve a 1V
offset, apply -1V to the OS_ pin to produce an output
range from 1V to (1V + V
REF x 2). Note that the DAC’s
output range is still limited by the maximum output volt-
age specification.
Power-Down Mode
The MAX5154/MAX5155 feature a software-program-
mable shutdown mode that reduces the typical supply
current to 2µA. The two DACs can be shutdown inde-
pendently, or simultaneously using the appropriate pro-
gramming command. Enter shutdown mode by writing
the appropriate input-control word (Table 1). In shut-
down mode, the reference inputs
and amplifier out-
puts become high impedance, and the serial
interface
remains active. Data in the input registers is
MAX5154/MAX5155
Low-Power, Dual, 12-Bit Voltage-Output DACs
with Serial Interface
_______________________________________________________________________________________ 9
Digital Ground DGND 9
Serial-Data Output DOUT 10
User-Programmable Output UPO 11
Power-Down Lockout. The device can-
not be powered down when PDL is low.
PDL 12
Reference for DAC B REFB 13
Active-Low Clear Input. Resets all reg-
isters to zero. DAC outputs go to 0V.
CL 5
Chip-Select Input CS 6
Serial-Data Input DIN 7
Serial Clock Input SCLK 8
Reference for DAC A REFA 4
DAC A Offset Adjustment OSA 3
PIN
DAC A Output Voltage OUTA 2
Analog Ground AGND 1
FUNCTION NAME
14 OSB DAC B Offset Adjustment
15 OUTB DAC B Output Voltage
16 V DD Positive Power Supply
OUT_
OS_
R
R
SHOWN FOR ALL 1s ON DAC
D0 D9 D10 D11
2R 2R 2R 2R 2R
R R R
REF_
AGND
Figure 1. Simplified DAC Circuit Diagram
_____________________Pin Description