AD648

REV. C

–7–

SIGN BIT

BINARY NUMBER IN DAC REGISTER ANALOG OUTPUT

0

1111 1111 1111

(4095/4096)

0

0000 0000 0000

0 VOLTS

1

0000 0000 0000

0 VOLTS

1

1111 1111 1111

(4095/4096)

NOTE: SIGN BIT AT "0" CONNECTS THE NONINVERTING INPUT OF

A2 TO ANALOG COMMON

Figure 25. Sign Magnitude Code Table

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.

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 circuit in Figure 24 provides 4 quadrant multiplication with

a resolution of 12 bits. The AD648 is used to convert the AD7545

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

perature, and excellent dynamics make it an attractive low

power 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:









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.

Figure 24. 12-Bit Plus Sign Magnitude D/A Converter