AD734
the AD734 can be operated using the standard (AD534) divider
connections (Figure 8), when the negative feedback path is
established via the Y
2
input. Substituting W for Y
2
in Equation
(2), we get
W
=
U
After temperature-correction (block TC), the reference voltage
is applied to transistor Qd and trimmed resistor Rd, which
generate the required reference current. Transistor Qu and
resistor Ru are not involved in setting up the internal denominator,
and their associated control pins U0, U1 and U2 will normally
be grounded. The reference voltage is also made available, via
the 100 kΩ resistor Rr, at Pin 9 (ER); the purpose of Qr is
explained below.
When the control pin DD (denominator disable) is connected to
VP, the internal source of Iu is shut off, and the collector cur-
rent of Qu must provide the denominator current. The resistor
Ru is laser-trimmed such that the multiplier denominator is
exactly equal to the voltage across it (that is, across pins U1 and
U2). Note that this trimming only sets up the correct internal
ratio; the absolute value of Ru (nominally 28 kΩ) has a
tolerance of
±
20%. Also, the alpha of Qu, (typically 0.995)
which might be seen as a source of scaling error, is canceled by
the alpha of other transistors in the complete circuit.
In the simplest scheme (Figure 3), an externally-provided
control voltage, V
G
, is applied directly to U0 and U2 and the
resulting voltage across Ru is therefore reduced by one V
BE
. For
example, when V
G
= 2 V, the actual value of U will be about
1.3 V. This error will not be important in some closed-loop
applications, such as automatic gain control (AGC), but clearly
is not acceptable where the denominator value must be well-
defined. When it is required to set up an accurate, fixed value of
U, the on-chip reference may be used. The transistor Qr is
provided to cancel the V
BE
of Qu, and is biased by an external
resistor, R2, as shown in Figure 4. R1 is chosen to set the de-
sired value of U and consists of a fixed and adjustable resistor.
(
Z
2
−
Z
1
)
+
Y
.
(
X
1
−
X
2
)
1
(5)
In this case, note that the variable X is now the denominator,
and the above restriction (X/U
≤
1.25) on the magnitude of the
X input does not apply. However, X must be positive in order
for the feedback polarity to be correct. Y
1
can be used for
summing purposes or connected to the load ground if not
needed. The shorthand form in this case is
±
Z
(
±W
)
=
(
+U
) ((
+
X
))
+
(
±Y
)
.
(6)
In some cases, feedback may be connected to two of the avail-
able inputs. This is true for the square-rooting connections
(Fig-ure 9), where W is connected to both X
1
and Y
2
. Setting
X
1
= W and Y
2
= W in Equation (2), and anticipating the
possibility of again providing a summing input, so setting X
2
= S
and Y
1
= S, we find, in shorthand form
(
±W
)
=
(
+U
)(
+
Z
)
+
(
±S
)
.
(7)
This is seen more generally to be the geometric-mean function,
since both U and Z can be variable; operation is restricted to
one quadrant. Feedback may also be taken to the U-interface.
Full details of the operation in these modes is provided in the
appropriate section of this data sheet.
Direct Denominator Control
A valuable new feature of the AD734 is the provision to replace
the internal denominator voltage, U, with any value from +10 mV
to +10 V. This can be used (1) to simply alter the multiplier
scaling, thus improve accuracy and achieve reduced noise levels
when operating with small input signals; (2) to implement an
accurate two-quadrant divider, with a 1000:1 gain range and an
asymptotic gain-bandwidth product of 200 MHz; (3) to achieve
certain other special functions, such as AGC or rms.
Figure 2 shows the internal circuitry associated with denomina-
tor control. Note first that the denominator is actually proportional
to a current, Iu, having a nominal value of 356
µA
for U = 10 V,
whereas the primary reference is a voltage, generated by a buried-
Zener circuit and laser-trimmed to have a very low temperature
coefficient. This voltage is nominally 8 V with a tolerance of
±
10%.
NOMINALLY
356 A for
U = 10V
14 VP
VP 14
Iu
3
U0
Qu
+V
S
AD734
DD 13
Rr
100k
ER
9 NC
~
60
A
V
G
NC 4
U1
Ru
28k
Qr
5
U2
VN
8
–V
S
Figure 3. Low-Accuracy Denominator Control
Iu
U0
3
U1
Ru
28k
R1
NC 5
U2
Qu
AD734
Rr
100k
VP 14
DD 13
R2
ER
Qr
9
+V
S
4
LINK TO
DISABLE
Iu
AD734
NOM
8V
VN
13 DD
U0 3
U1 4
Ru
28k
U2
5
NEGATIVE SUPPLY
Rd
NOM
22.5k
Qu
Qd
TC
Qr
NOM
8V
8 VN
Rr
100k
9 ER
8
–V
S
Figure 4. Connections for a Fixed Denominator
Table I shows useful values of the external components for set-
ting up nonstandard denominator values.
Figure 2. Denominator Control Circuitry
REV. C
–5–
AD [ ANALOG DEVICES ]
