Detailed description – Rainbow Electronics MAX5189 User Manual

Page 8

MAX5186/MAX5189

Dual, 8-Bit, 40MHz, Current/Voltage,
Simultaneous-Output DACs

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Detailed Description

The MAX5186/MAX5189 are dual, 8-bit digital-to-ana-
log converters (DACs) capable of operating with clock
speeds up to 40MHz. Each of these dual converters
consists of separate input and DAC registers, followed
by a current source array capable of generating up to
1.5mA full-scale output current (Figure 1). An integrat-
ed +1.2V voltage reference and control amplifier deter-
mine the data converters’ full-scale output currents/
voltages. Careful reference design ensures close gain
matching and excellent drift characteristics. The
MAX5189’s voltage output operation features matched
400

Ω on-chip resistors that convert the current array

current into a voltage.

Internal Reference and

Control Amplifier

The MAX5186/MAX5189 provide an integrated
50ppm/°C, +1.2V, low-noise bandgap reference that
can be disabled and overridden by an external refer-
ence voltage. REFO serves either as an external refer-
ence input or an integrated reference output. If REN is
connected to DGND, the internal reference is selected
and REFO provides a +1.2V output. Due to its limited
10µA output drive capability, REFO must be buffered
with an external amplifier if heavier loading is required.

The MAX5186/MAX5189 also employ a control amplifier
designed to simultaneously regulate the full-scale out-
put current (I

) for both outputs of the devices. The

output current is calculated as follows:

= 8

REF

where I

REF

is the reference output current (I

REF

REFO

SET

) and I

is the full-scale output current.

SET

is the reference resistor that determines the

amplifier’s output current on the MAX5186 (Figure 2).
This current is mirrored into the current source array
where it is equally distributed between matched current
segments and summed to valid output current readings
for the DACs.

The MAX5189 converts each output current (DAC1 and
DAC2) into an output voltage (V

OUT1

, V

OUT2

) with two

internal, ground-referenced 400

Ω load resistors. Using

the internal +1.2V reference voltage, the MAX5189’s
integrated reference output-current resistor (R

SET

9.6k

Ω) sets I

REF

to 125µA and I

to 1mA.

External Reference

To disable the MAX5186/MAX5189’s internal reference,
connect REN to DV

. A temperature-stable, external

reference may now be applied to drive the REFO pin to
set the full-scale output (Figure 3). Choose a reference
capable of supplying at least 150µA to drive the bias
circuit that generates the cascode current for the cur-
rent array. For improved accuracy and drift perfor-
mance, choose a fixed output voltage reference such
as the +1.2V, 25ppm/°C MAX6520 bandgap reference.

Standby Mode

To enter the lower power standby mode, connect digi-
tal inputs PD and DACEN to DGND. In standby, both
the reference and the control amplifier are active with
the current array inactive. To exit this condition, DACEN
must be pulled high with PD held at DGND. Both the
MAX5186/MAX5189 typically require 50µs to wake up
and let both outputs and reference settle.

Shutdown Mode

For lowest power consumption, the MAX5186/MAX5189
provide a power-down mode in which the reference,
control amplifier, and current array are inactive and the
DACs’ supply current is reduced to 1µA. To enter this
mode, connect PD to DV

. To return to active mode,

connect PD to DGND and DACEN to DV

. About 50µs

are required for the parts to leave shutdown mode and
settle to their outputs’ values prior to shutdown. Table 1
lists the power-down mode selection.

Timing Information

Both DAC cells residing in the MAX5186/MAX5189
write to their outputs simultaneously (Figure 4). The
input latch of the first DAC (DAC1) is loaded after the
clock signal transitions high. When the clock signal
transitions low, the input latch of the second DAC
(DAC2) is loaded. The contents of both input latches
are simultaneously shifted to the DAC registers, and
their outputs update at the rising edge of the next
clock.

Outputs

The MAX5186 outputs are designed to supply full-scale
output currents of 1mA into 400

Ω loads in parallel with

a capacitive load of 5pF. The MAX5189 features inte-
grated 400

Ω resistors that restore the array currents to

proportional, differential voltages of 400mV. These dif-
ferential output voltages can then be used to drive a
balun transformer or a low-distortion, high-speed oper-
ational amplifier to convert the differential voltage into a
single-ended voltage.