AD7538 Analog Devices, AD7538 Datasheet - Page 12

• Manufacturer Part Number: AD7538
• Description: LC2MOS Microprocessor-Compatible 14-Bit DAC
• Manufacturer: Analog Devices

Specifications of AD7538

• Resolution (bits): 14bit
• Dac Update Rate: 667kSPS
• Dac Settling Time: 1.5µs
• Max Pos Supply (v): +15.75V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Par

AD7538
PROGRAMMABLE GAIN AMPLIFIER
The circuit shown in Figure 9 provides a programmable gain
amplifier (PGA). In it the DAC behaves as a programmable
resistance and thus allows the circuit gain to be digitally
controlled.
The transfer function of Figure 9 is:
R
V
where:
n is the resolution of the DAC.
N is the DAC input code in decimal.
R
EQ
IN
REF
is the constant input impedance of the DAC (R
is the equivalent transfer impedance of the DAC from the
pin to the I
Gain
R
EQ
NOTES
1. RESISTOR R
V
IN
=
INCLUDED ON THE DICE.
=
2
V
Figure 9. Programmable Gain Amplifier (PGA)
V
n
OUT
N
R
R
IN
FB
OUT
IN
=
pin and can be expressed as
FB
−
IS ACTUALLY
R
R
A
EQ
FB
A
I
GND
OUT
V
DIGITAL
V
INPUT
AD7538
DD
SS
N
V
V
REF
DD
V
IN
OUT
= R
LAD
).
Rev. B | Page 12 of 16
(1)
(2)
Substituting this expression into Equation 1 and assuming
zero gain error for the DAC (R
simplifies to
The ratio N/2
such, is the fractional representation of the digital input word.
Equation 4 indicates that the gain of the circuit can be varied
from 16,384 down to unity (actually 16,384/16,383) in 16,383
steps. The all 0s code is never applied. This avoids an open-loop
condition thereby saturating the amplifier. With the all 0s code
excluded there remains (2
choice of (2
20
V
V
V
V
OUT
OUT
IN
IN
log
10
=
=
n
– 1) output levels. In decibels the dynamic range is
−
−
n
V
V
is commonly represented by the term, D, and, as
OUT
2
−
N
IN
N
n
2
n
=
=
20
−
D
1
log
n
– 1) possible input codes allowing a
10
(
2
IN
n
= R
−
1
)
FB
=
), the transfer function
84
dB
(3)
(4)