LTC2704-14 Linear Technology Corporation, LTC2704-14 Datasheet - Page 17

• Manufacturer Part Number: LTC2704-14
• Description: Ltc2704-14 - Quad 14-bit Voltage Output Softspan Dac With Readback
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Overview
The LTC2704 is a highly integrated device, greatly simpli-
fying design and layout as compared to a design using
multiple current output DACs and separate amplifiers. A
similar design using four separate current output DACs
would require six precision op amps, compensation ca-
pacitors, bypass capacitors for each amplifier, several
times as much PCB area and a more complicated serial
interface. Still, it is important to avoid some common
mistakes in order to achieve full performance. DC752A is
the evaluation board for the LTC2704. It is designed to
meet all data sheet specifications, and to allow the LTC2704
to be integrated into other prototype circuitry. All force/
sense lines are available to allow the addition of current
booster stages or other output circuits.
The DC752A design is presented as a tutorial on properly
applying the LTC2704. This board shows how to properly
return digital and analog ground currents, and how to
compensate for small differences in ground potential
between the two banks of two DACs. There are other ways
to ground the LTC2704, but the one requirement is that
analog and digital grounds be connected at the LTC2704
by a very low impedance path. It is NOT advisable to split
the ground plane and connect them with a jumper or
inductor. When in doubt, use a single solid ground plane
rather than separate planes.
The LTC2704 does allow the ground potential of the DACs
to vary by ±300mV with respect to analog ground, allow-
ing compensation for ground return resistance.
Power Supply Grounding and Noise
LTC2704 V
amplifiers, ground sense amplifiers and reference inver-
sion amplifiers. These amplifiers have good power supply
rejection, but the V
wideband noise. The best scheme is to prefilter low noise
regulators such as the LT
(negative). Refer to Linear Technology Application Note
101, Minimizing Switching Regulator Residue in Linear
Regulator Outputs.
+
and V
–
U
pins are the supplies to all of the output
+
and V
U
®
–
1761 (positive) and LT1964
supplies must be free from
W
U
The LTC2704 V
analog DAC switches and is very sensitive to noise. It must
be treated as an analog supply. The evaluation board uses
an LT1790 precision reference as the V
mize noise.
The GND pin is the return for digital currents and the AGND
pin is a bias point for internal analog circuitry. Both of these
pins must be tied to the same point on a quiet ground plane.
Each DAC has a separate ground sense pin that can be
used to compensate for small differences in ground poten-
tial within a system. Since DACs A and B are associated
with REF1 and DACs C and D are associated with REF2, the
grounds must be grouped together as follows:
AGNDA, AGNDB and REFG1 tied together (“GND1” on
DC752A)
AGNDC, AGNDD and REFG2 tied together (“GND2” on
DC752A)
This scheme allows compensation for ground return IR
drops, as long as the resistance is shared by both DACs in
a group. This implies that the ground return for DACs A
and B must be as close as possible, and GND1 must be
connected to this point through a low current, low resis-
tance trace. (Similar for DACs C and D.)
Figure 3 shows the top layer of the evaluation board. The
GND1 trace connects REFG1, AGNDA, AGNDB and the
ground pin of the LT1236 precision reference (U4.) This
point is the ground reference for DACs A and B. The GND2
trace connects REFG2, AGNDC, AGNDD and the ground
pin of the other LT1236 precision reference (U5). This
point is the ground reference for DACs C and D.
Voltage Reference
A high quality, low noise reference such as the LT1236 or
LT1027 must be used to achieve full performance. The
ground terminal of this reference must be connected
directly to the common ground point. If GND1 and GND2
are separate, then two references must be used.
DD
pin is the supply for the digital logic and
DD
LTC2704
supply to mini-
17
2704f