Datasheet AD624 (Analog Devices) - 12

AD624 50
⍀
–IN 1 16 OUTPUT 50 G = 100
⍀
OFFSET G = 200 G = 500 +IN 2 80.2
⍀
15 TRIM K1 K2 K3 NC R2 3 4445.7
⍀
14 10k
⍀
INPUT OFFSET 4 13 TRIM 20k
⍀
VB 20k
⍀
225.3
⍀
RELAY SHIELDS R1 5 12 10k
⍀
10k
⍀
10k
⍀
10k 124
⍀ ⍀
6 11 +5V 10k
⍀
–V 7 10 S K1 K2 K3 AD624 +V 8 9 D1 D2 D3 S OUT C1 C2 1
␮
F 35V K1 – K3 = ANALOG THERMOSEN DM2C COMMON 4.5V COIL INPUTS A D1 – D3 = IN4148 GAIN Y0 RANGE B 74LS138 Y1 7407N DECODER BUFFER 10
␮
F GAIN TABLE Y2 DRIVER A B GAIN 0 0 100 0 1 500 1 0 200 +5V 1 1 1 LOGIC COMMON
Figure 38. Gain Programmable Amplifier By establishing a reference at the “low” side of a current setting symmetrical bipolar transmission is ideal in this application. The resistor, an output current may be defined as a function of input multiplying DAC’s advantage is that it can handle inputs of voltage, gain and the value of that resistor. Since only a small either polarity or zero without affecting the programmed gain. current is demanded at the input of the buffer amplifier A2, the The circuit shown uses an AD7528 to set the gain (DAC A) and forced current I to perform a fine adjustment (DAC B). L will largely flow through the load. Offset and drift specifications of A2 must be added to the output offset and drift specifications of the IA.
(+INPUT) 50
⍀
–IN 1 16 OUTPUT PROGRAMMABLE GAIN (–INPUT) 50
⍀
OFFSET NULL
Figure 38 shows the AD624 being used as a software program-
+IN 2 80.2
⍀
15 TO –V
mable gain amplifier. Gain switching can be accomplished with
3 14 10k
⍀ mechanical switches such as DIP switches or reed relays. It
INPUT 4445.7
⍀
OFFSET
should be noted that the “on” resistance of the switch in series
4 13 NULL 20k
⍀
VB 20k
⍀
225.3
⍀ with the internal gain resistor becomes part of the gain equation
5 12 10k
⍀ and will have an effect on gain accuracy.
10k
⍀
10k
⍀
124
⍀
6 11
A significant advantage in using the internal gain resistors in a
10k
⍀
10k
⍀ programmable gain configuration is the minimization of thermo-
–V 7 10 S AD624
couple signals which are often present in multiplexed data
+V 8 9 S VOUT
acquisition systems.
1
␮
F 35V
If the full performance of the AD624 is to be achieved, the user
10pF V V SS DD GND
must be extremely careful in designing and laying out his circuit to minimize the remaining thermocouple signals.
+VS
The AD624 can also be connected for gain in the output stage.
39.2k
⍀
1k
⍀ Figure 39 shows an AD547 used as an active attenuator in the
AD711 28.7k
⍀
1k
⍀ output amplifier’s feedback loop. The active attenuation pre-
–VS 316k
⍀
1k
⍀ sents a very low impedance to the feedback resistors therefore
AD7590
minimizing the common-mode rejection ratio degradation. Another method for developing the switching scheme is to use a
A1 A2 A3 A4 WR
DAC. The AD7528 dual DAC which acts essentially as a pair of switched resistive attenuators having high analog linearity and Figure 39. Programmable Output Gain REV. C –11–