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AD7845BQ

P1-P3P4-P6P7-P9P10-P12P13-P13
AD7845
REV. B
–6–
PIN FUNCTION DESCRIPTION (DIP)
Pin Mnemonic Description
1V
OUT
Voltage Output Terminal
2-11 DB11–DB2 Data Bit 11 (MSB) to Data Bit 2
12 DGND Digital Ground. The metal lid on the ceramic package is connected to this pin
13-14 DB1–DB0 Data Bit 1 to Data Bit 0 (LSB)
15 WR Write Input. Active low
16 CS Chip Select Input. Active low
17 V
REF
Reference Input Voltage which can be an ac or dc signal
18 AGND Analog Ground. This is the reference point for external analog circuitry
19 V
SS
Negative power supply for the output amplifier (nominal –12 V to +15 V)
20 V
DD
Positive power supply (nominal +12 V to +15 V)
21 R
A
Application resistor. R
A
= 4 R
FB
22 R
B
Application resistor. R
B
= 2 R
FB
23 R
C
Application resistor. R
C
= 2 R
FB
24 R
FB
Feedback resistor in the DAC. For normal operation this is connected to V
OUT
CIRCUIT INFORMATION
Digital Section
Figure 11 is a simplified circuit diagram of the AD7845 input
control logic. When CS and WR are both low, the DAC latch is
loaded with the data on the data inputs. All the digital inputs
are TTL, HCMOS and +5 V CMOS compatible, facilitating
easy microprocessor interfacing. All digital inputs incorporate
standard protection circuitry.
Figure 11. AD7845 Input Control Logic
D/A Section
Figure 12 shows a simplified circuit diagram for the AD7845
D/A section and output amplifier.
A segmented scheme is used whereby the 2 MSBs of the 12-bit
data word are decoded to drive the three switches A-C. The
remaining 10 bits drive the switches (S0–S9) in a standard R-2R
ladder configuration.
RRR
2R
2R
2R
2R 2R
BA S9
2R
C
V
REF
S8 S0
SHOWN FOR ALL 1s ON DAC
2R
I
OUT
AGND
Figure 12. Simplified Circuit Diagram for the AD7845 D/A
Section
Each of the switches A–C steers 1/4 of the total reference cur-
rent with the remaining 1/4 passing through the R-2R section.
An output amplifier and feedback resistor perform the current-
to-voltage conversion giving
V
OUT
= – D × V
REF
where D is the fractional representation of the digital word. (D
can be set from 0 to 4095/4096.)
The amplifier can maintain ±10 V across a 2 kΩ load. It is inter-
nally compensated and settles to 0.01% FSR (1/2 LSB) in less
than 5 µs. The input offset voltage is laser trimmed at wafer
level. The amplifier slew rate is typically 11 V/µs, and the unity
gain small signal bandwidth is 600 kHz. There are three extra
on-chip resistors (R
A
, R
B
, R
C
) connected to the amplifier invert-
ing terminal. These are useful in a number of applications in-
cluding offset adjustment and gain ranging.
AD7845
REV. B –7–
UNIPOLAR BINARY OPERATION
Figure 13 shows the AD7845 connected for unipolar binary
operation. When V
IN
is an ac signal, the circuit performs
2-quadrant multiplication. The code table for Figure 13 is given
in Table I.
Figure 13. Unipolar Binary Operation
Table I. Unipolar Binary Code Table for AD7845
Binary Number In
DAC Register Analog Output, V
OUT
MSB LSB
1111 1111 1111 –V
IN
4095
4096
1000 0000 0000 –V
IN
2048
4096
= –1/2 V
IN
0000 0000 0001 –V
IN
1
4096
0000 0000 0000 0 V
OFFSET AND GAIN ADJUSTMENT FOR FIGURE 13
Zero Offset Adjustment
1. Load DAC with all 0s.
2. Trim R3 until V
OUT
= 0 V.
Gain Adjustment
1. Load DAC with all 1s.
2. Trim R1 so that V
OUT
= –V
IN
4095
4096
.
In fixed reference applications, full scale can also be adjusted by
omitting R1 and R2 and trimming the reference voltage magni-
tude. For high temperature applications, resistors and potenti-
ometers should have a low temperature coefficient.
BIPOLAR OPERATION
(4-QUADRANT MULTIPLICATION)
The recommended circuit for bipolar operation is shown in
Figure 14. Offset binary coding is used.
The offset specification of this circuit is determined by the
matching of internal resistors R
B
and R
C
and by the zero code
offset error of the device. Gain error may be adjusted by varying
the ratio of R1 and R2.
To use this circuit without trimming and keep within the gain
error specifications, resistors R1 and R2 should be ratio
matched to 0.01%.
The code table for Figure 14 is given in Table II.
Figure 14. Bipolar Offset Binary Operation
Table II. Bipolar Code Table for Offset Binary Circuit of
Figure 14
Binary Number In
DAC Register Analog Output, V
OUT
MSB LSB
1111 1111 1111 +V
IN
2047
2048
1000 0000 0001 +V
IN
1
2048
1000 0000 0000 0 V
0111 1111 1111 –V
IN
1
2048
0000 0000 0000 –V
IN
2048
2048
= –V
IN
AD7845
REV. B
–8–
APPLI
CATION
S CIRCUITS
PROGRAMMABLE GAIN AMPLIFIER (PGA)
The AD7845 performs a PGA function when connected as in
Figure 15. In this configuration, the R-2R ladder is connected
in the amplifier feedback loop. R
FB
is the amplifier input resis-
tor. As the code decreases, the R-2R ladder resistance increases
and so the gain increases.
V
OUT
= –V
IN
×
R
DAC
D
×
1
R
FB
,
D = 0 to
4095
4096
= –V
IN
×
R
DAC
D
×
1
R
DAC
=
–V
IN
D
, since R
FB
= R
DAC
Figure 15. AD7845 Connected as PGA
As the programmed gain increases, the error and noise also
increase. For this reason, the maximum gain should be limited
to 256. Table III shows gain versus code.
Note that instead of using R
FB
as the input resistor, it is also
possible to use combinations of the other application resistors,
R
A
, R
B
and R
C
. For instance, if R
B
is used instead of R
FB
, the
gain range for the same codes of Table II now goes from l/2
to 128.
Table III. Gain and Error vs. Input Code for Figure 15
Digital Inputs Gain Error (%)
1111 1111 1111 4096/4095 ≈ 10.04
1000 0000 0000 2 0.07
0100 0000 0000 4 0.13
0010 0000 0000 8 0.26
0001 0000 0000 16 0.51
0000 1000 0000 32 1.02
0000 0100 0000 64 2.0
0000 0010 0000 128 4.0
0000 0001 0000 256 8.0
PROGRAMMABLE CURRENT SOURCES
The AD7845 is ideal for designing programmable current
sources using a minimum of external components. Figures 16
and 17 are examples. The circuit of Figure 16 drives a program-
mable current I
L
into a load referenced to a negative supply.
Figure 17 shows the circuit for sinking a programmable current,
I
L
. The same set of circuit equations apply for both diagrams.
I
L
= I
3
= I
2
+ I
1
I
1
=
D ×|V
IN
|
R
DAC
,
D = 0 to
4095
4096
I
2
=
1
R1
D ×|V
IN
|
R
DAC
R
FB
=
D ×|V
IN
|
R1
, since R
FB
= R
DAC
I
L
=
D ×|V
IN
|
R1
+
D ×|V
IN
|
R
DAC
=
D ×|V
IN
|
R1
×
1 +
R1
R
DAC
Note that by making R1 much smaller than R
DAC
, the circuit
becomes insensitive to both the absolute value of R
DAC
and its
temperature variations. Now, the only resistor determining load
current I
L
is the sense resistor R1.
If R1 = 100 Ω, then the programming range is 0 mA to 100 mA,
and the resolution is 0.024 mA.
Figure 16. Programmable Current Source
P1-P3P4-P6P7-P9P10-P12P13-P13

AD7845BQ

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Manufacturer:
Analog Devices Inc.
Description:
Digital to Analog Converters - DAC 12B CMOS Multiplying
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