AD648
Figure 23c shows a diode clamp protection scheme for an I-to-V
converter using low leakage diodes. Because the diodes are con-
nected to the op amp’s summing junction, which is a virtual
ground, their leakage contribution is minimal.
CMOS DAC’s output current to a voltage and provides the
necessary level shifting to achieve a bipolar voltage output. The
circuit operates with a 12-bit plus sign input code. The transfer
function is shown in Figure 25.
The AD7592 is a fully protected dual CMOS SPDT switch with
data latches. R4 and R5 should match to within 0.01% to main-
tain the accuracy of the converter. A mismatch between R4 and
R5 introduces a gain error. Overall gain is trimmed by adjusting
R
IN
. The AD648’s low input offset voltage, low drift over tem-
perature, and excellent dynamics make it an attractive low
power output buffer.
Figure 23c. I-to-V Converter with Diode Input Protection
Exceeding the negative common-mode range on either input
terminal causes a phase reversal at the output, forcing the ampli-
fier output to the corresponding high or low state. Exceeding
the negative common mode on both inputs simultaneously
forces the output high. Exceeding the positive common-mode
range on a single input doesn’t cause a phase reversal; but if
both inputs exceed the limit, the output will be forced high. In
all cases, normal amplifier operation is resumed when input
voltages are brought back within the common-mode range.
D/A CONVERTER BIPOLAR OUTPUT BUFFER
The input offset voltage of the AD648 output amplifier results
in an output error voltage. This error voltage equals the input
offset voltage of the op amp times the noise gain of the amplifier.
That is:
R
V
OS
Output
=
V
OS
Input
1
+
FB
R
O
R
FB
is the feedback resistor for the op amp, which is internal to
the DAC. R
O
is the DAC’s R-2R ladder output resistance. The
value of R
O
is code dependent. This has the effect of changing
the offset error voltage at the amplifier’s output. An output am-
plifier with a sub millivolt input offset voltage is needed to pre-
serve the linearity of the DAC’s transfer function.
The circuit in Figure 24 provides 4 quadrant multiplication with
a resolution of 12 bits. The AD648 is used to convert the AD7545
Figure 24. 12-Bit Plus Sign Magnitude D/A Converter
SIGN BIT
0
0
1
1
BINARY NUMBER IN DAC REGISTER ANALOG OUTPUT
1111
0000
0000
1111
1111
0000
0000
1111
1111
0000
0000
1111
+V
IN
(4095/4096)
0 VOLTS
0 VOLTS
–V
IN
(4095/4096)
NOTE: SIGN BIT AT "0" CONNECTS THE NONINVERTING INPUT OF
A2 TO ANALOG COMMON
Figure 25. Sign Magnitude Code Table
REV. C
–7–
AD [ ANALOG DEVICES ]
