Rainbow Electronics MAX5177 - Manual (Page 10)

MAX5175/MAX5177
Low-Power, Serial, 12-Bit DACs with
Force/Sense Voltage Output
10 ______________________________________________________________________________________
Detailed Description
The MAX5175/MAX5177 12-bit, serial, voltage-output
DACs operate with a 3-wire serial interface. These
devices include a 16-bit shift register and a double-
buffered input composed of an input register and a
DAC register (see
Functional Diagram ). In addition, the
negative terminal of the output amplifier is available.
The DACs are designed with an inverted R-2R ladder
network (Figure 1) that produces a weighted voltage
proportional to the reference voltage.
Reference Inputs
The reference input accepts both AC and DC values with
a voltage range extending from 0 to V
DD - 1.4V. The fol-
lowing equation represents the resulting output voltage:
where N is the numeric value of the DAC’s binary input
code (0 to 4095), V
REF is the reference voltage, and
Gain is the externally set voltage gain. The maximum
output voltage is V
DD . The reference pin has a mini-
mum impedance of 18k
Ω
and is code dependent.
Output Amplifier
The MAX5175/MAX5177’s DAC output is internally
buffered by a precision amplifier with a typical slew rate
of 0.6V/µs. Access to the output amplifier’s inverting
input provides flexibility in output gain setting and sig-
nal conditioning (see
Applications Information ).
The output amplifier settles to ±0.5LSB from a full-scale
transition within 12µs, when loaded with 5k
Ω
in parallel
with 100pF. Loads less than 2k
Ω
degrade perfor-
mance.
Shutdown Mode
The MAX5175/MAX5177 feature a software- and hard-
ware-programmable shutdown mode that reduces the
typical supply current to 1µA. Enter shutdown by writing
the appropriate input-control word as shown in Table 1
or by using the hardware shutdown function. In shut-
down mode, the reference input and the amplifier out-
put become high impedance and the serial interface
remains active. Data in the input register is saved,
allowing the MAX5175/MAX5177 to recall the prior out-
put state when returning to normal operation. Exit shut-
down by reloading the DAC register from the shift
register, by simultaneously loading the input and DAC
registers, or by toggling PDL. When returning from
shutdown, wait 40µs for the output to settle.
Power-Down Lockout
Power-down lockout disables the software/hardware
shutdown mode. A high-to-low transition on PDL brings
the device out of shutdown, returning the output to its
previous state.
Shutdown
Pulling SHDN high while PDL is high places the
MAX5175/MAX5177 in shutdown. Pulling SHDN low does
not take the device out of shutdown. A high-to-low transi-
tion on PDL or an appropriate command from the serial
data line (see Table 1 for commands) is required to exit
shutdown.
Serial Interface
The 3-wire serial interface is compatible with SPI, QSPI
(Figure 2), and MICROWIRE (Figure 3) interface stan-
dards. The 16-bit serial input word consists of two con-
trol bits, 12 bits of data (MSB to LSB), and two sub-bits.
The control bits determine the MAX5175/MAX5177’s
response as outlined in Table 1. The digital inputs are
double buffered, which allows any of the following:
•
Loading the input register without updating the DAC
register
•
Updating the DAC register from the input register
•
Updating the input and DAC registers simultaneously.
V
V N GAIN
4096
OUT
REF =
⋅ ⋅
OUT
FB
SHOWN FOR ALL 1s ON DAC
MSB
2R 2R 2R 2R 2R
R R R
REF
AGND
Figure 1. Simplified DAC Circuit Diagram