AD876
Figure 16 shows the equivalent input structure for the AD876
reference pins. There is approximately 5
Ω
of resistance between
both the REFTF and REFBT pins and the reference ladder. If
the force-sense connections are not used, the voltage drop
across the 5
Ω
resistors will result in a reduced voltage appear-
ing across the ladder resistance. This reduces the input span of
the converter. Applying a slightly larger span between the REFTF
and REFBF pins compensates this error. Note that the tem-
perature coefficients of the 5
Ω
resistors are 1350 ppm. The
user should consider the effects of temperature when not using
a force-sense reference configuration.
5
ance changes associated with the reference inputs. The simpli-
fied diagram of Figure 16 shows that the reference pins connect
to a capacitor for one-half of the clock period. The size of the
capacitor is a function of the analog input voltage.
The external reference must be able to maintain a low imped-
ance over all
frequencies of interest
in order to provide the charge
required by the capacitance. By supplying the requisite charge,
the reference voltages will be relatively constant and perfor-
mance will not degrade. For some reference configurations,
voltage transients will be present on the reference lines; this
is particularly true during the falling edge of CLK. It is impor-
tant that the reference recovers from the transients and settles to
the desired level of accuracy prior to the rising edges of CLK.
There are several reference configurations suitable for the
AD876 depending on the application, desired level of accuracy,
and cost trade-offs. The simplest configuration, shown in Fig-
ure 18, utilizes a resistor string to generate the reference volt-
ages from the converter’s analog power supply. The 0.1
µF
bypass capacitors effectively reduce high-frequency transients.
The 10
µF
capacitors act to reduce the impedances at the
REFTF and REFBF pins at lower frequencies. As input fre-
quencies approach dc, the capacitors become ineffective, and
small voltage deviations will appear across the biasing resistors.
This application can maintain 10-bit accuracy for input frequen-
cies above approximately 200 Hz. 8-bit applications can use this
circuit for input frequencies above approximately 50 Hz.
AD876
NC
140
+5V
10 F
10 F
250
( 1%)
10 F
2V
REFBF
0.1 F
( 1%)
REFTS
REFTF
REFTS
AD876
V1
DACS
CLK
C (V
IN
)
CLK
R
LADDER
250
V2
REFBS
REFBF
5
Figure 16. AD876 Equivalent Reference Structure
Do not connect the REFTS and REFBS pins in configurations
that do not use a force-sense reference. Connecting the force
and sense lines together allows current to flow in the sense lines.
Any current allowed to flow through these lines must be negligi-
bly small. Current flow causes voltage drops across the resis-
tance in the sense lines. Because the internal D/As of the
AD876 tap different points along the sense lines, each D/A
would receive a slightly different reference voltage if current
were flowing in these wires. To avoid this undesirable condition,
leave the sense lines unconnected. Any current allowed to flow
through these lines must be negligibly small (<100
µA).
The voltage drop across the internal resistor ladder determines
the input span of the AD876. The driving voltages required at
the V1 and V2 points are respectively +4 V and +2 V. Calculate
the full-scale input span from the equation
Input Span
(V )
=
REFTS
–
REFBS
4V
0.1 F
REFTF
250
( 15%)
NC
NC = NO CONNECT
REFBS
This results in a full-scale input span of approximately +2 V
when REFTS = +4 V and REFBS = +2 V In order to maintain
the requisite 2 V drop across the internal ladder, the external
reference must be capable of providing approximately 8.0 mA.
The user has flexibility in determining both the full-scale span of
the analog input and where to center this voltage. Figure 17
shows the range over which the AD876 can operate without
degrading the typical performance.
(1.6, 4.5)
4.5
REFTF, REFTS
4.0
3.5
(1.6, 3.5)
3.0
2.5
1.0
1.5
2.0
2.5
REFBF, REFBS
3.0
(2.5, 3.5)
(2.5, 4.5)
Figure 18. Low Cost Reference Circuit
This reference configuration provides the lowest cost but has
several disadvantages. These disadvantages include poor dc
power supply rejection and poor accuracy due to the variability
of the internal and external resistors.
The AD876 offers force-sense reference connections to elimi-
nate the voltage drops associated with the internal connections
to the reference ladder. Figure 19 shows a suggested circuit
using an AD826 dual, high speed op amp. This configuration
uses 3.6 V and 1.6 V reference voltages for REFT and REFB,
respectively. The connections shown in Figure 19 configure the
op amps as voltage followers.
Figure 17. AD876 Reference Ranges
While the previous issues address the dc aspects of the AD876
reference, the user must also be aware of the dynamic imped-
REV. B
–9–
AD [ ANALOG DEVICES ]
