LTC1657 Ver la hoja de datos (PDF) - Linear Technology

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LTC1657

Parallel 16-Bit Rail-to-Rail Micropower DAC

Linear Technology 

LTC1657/LTC1657L

U

OPERATIO

Parallel Interface

The data on the input of the DAC is written into the DAC’s

input registers when Chip Select (CSLSB and/or CSMSB)

and WR are at a logic low. The data that is written into the

input registers will depend on which of the Chip Selects

are at a logic low (see Digital Interface Truth Table). If WR

and CSLSB are both low and CSMSB is high, then only

data on the eight LSBs (D0 to D7) is written into the input

registers. Similarly, if WR and CSMSB are both low and

CSLSB is high, then only data on the eight MSBs (D8 to

D15) is written into the input registers. Data is written into

both the Least Significant Data Bits (D0 to D7) and the Most

Significant Bits (D8 to D15) at the same time if WR, CSLSB

and CSMSB are low. If WR is high or both CSMSB and

CSLSB are high, then no data is written into the input

registers.

Once data is written into the input registers, it can be

written into the DAC register. This will update the analog

voltage output of the DAC. The DAC register is written by

a logic low on LDAC. The data in the DAC register will be

held when LDAC is high.

When WR, CSLSB, CSMSB and LDAC are all low, the

registers are transparent and data on pins D0 to D15 flows

directly into the DAC register.

For an 8-bit data bus connection, tie the MSB byte data

pins to their corresponding LSB byte pins (D15 to D7, D14

to D6, etc).

Power-On Reset

The LTC1657/LTC1657L have an internal power-on reset

that resets all internal registers to 0’s on power-up and

VOUT pin forces to GND (equivalent to the CLR pin

function).

Reference

The LTC1657/LTC1657L include an internal 2.048V/1.25V

reference, giving the LTC1657/LTC1657L a full-scale range

of 4.096V/2.5V in the gain-of-2 configuration. The onboard

reference in the LTC1657/LTC1657L is not internally

connected to the DAC’s reference resistor string but is

provided on an adjacent pin for flexibility. Because the

internal reference is not internally connected to the DAC

resistor ladder, an external reference can be used or the

resistor ladder can be driven by an external source in

multiplying applications. The external reference or source

must be capable of driving the 16k (minimum) DAC ladder

resistance.

Internal reference output noise can be reduced with a

bypass capacitor to ground. (Note: The reference does not

require a bypass capacitor to ground for nominal opera-

tion.) When bypassing the reference, a small value resis-

tor in series with the capacitor is recommended to help

reduce peaking on the output. A 10Ω resistor in series

with a 4.7µF capacitor is optimum for reducing reference

generated noise. Internal reference output voltage noise

spectral density at 1kHz is typically 150nV/√Hz (LTC1657),

90nV/√Hz (LTC1657L)

DAC Resistor Ladder

The high and low end of the DAC ladder resistor string

(REFHI and REFLO, respectively) are not connected inter-

nally on this part. Typically, REFHI will be connected to

REFOUT and REFLO will be connected to GND. X1/X2

connected to GND will give the LTC1657/LTC1657L a full-

scale output swing of 4.096V/2.5V.

Either of these pins can be driven up to VCC – 1.5V when

using the buffer in the gain-of-1 configuration. The resistor

string pins can be driven to VCC/2 when the buffer is in the

gain of 2 configuration. The resistance between these two

pins is typically 25k (16k min) (LTC1657), 23k (16k min)

(LTC1657L).

Voltage Output

The output buffer for the LTC1657/LTC1657L can be

configured for two different gain settings. By tying the

X1/X2 pin to GND, the gain is set to 2. By tying the X1/X2

pin to VOUT, the gain is set to unity.

The LTC1657/LTC1657L rail-to-rail buffered output can

source or sink 5mA within 500mV of the positive supply

voltage or ground at room temperature. The output stage

is equipped with a deglitcher that results in a midscale

glitch impulse of 8nV • s. The output swings to within a few

millivolts of either supply rail when unloaded and has an

equivalent output resistance of 40Ω (LTC1657), 120Ω

(LTC1657L) when driving a load to the rails.

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