Datasheet AD688 (Analog Devices) - 7

link to page 7 link to page 7 link to page 7 link to page 7
AD688 APPLICATIONS
The AD688 can be configured to provide ±10 V reference voltage and the position of the center tap within the span. The outputs as shown in Figure 4. The architecture of the AD688 gain adjustment should be performed first. Although the trims provides ground sense and uncommitted output buffer are not interactive within the device, the gain trim will move the amplifiers which offer the user a great deal of functional balance trim point as it changes the magnitude of the span. flexibility. The AD688 is specified and tested in the Figure 5 shows the gain and balance trims of the AD688. A configuration shown in Figure 4. The user may choose to take 100 kΩ 20-turn potentiometer is used for each trim. The advantage of other configuration options available with the potentiometer for the gain trim is connected between Pin 6 AD688; however performance in these configurations is not (VHIGH) and Pin 8 (VLOW) with the wiper connected to Pin 5 guaranteed to meet the stringent data sheet specifications. (GAIN ADJ). The potentiometer is adjusted to produce exactly Unbuffered outputs are available at Pin 6 and Pin 8. Loading of 20 V between Pin 1 and Pin 15, the amplifier outputs. The these unbuffered outputs will impair circuit performance. balance potentiometer, also connected between Pin 6 and Pin 8 with the wiper to Pin 12 (BAL ADJ), is then adjusted to center Amplifiers A3 and A4 can be used interchangeably. However, the span from +10 V to −10 V. the AD688 is tested (and the specifications are guaranteed) with the amplifiers connected as indicated in Figure 4. When either Input impedance on both the GAIN ADJ and the BAL ADJ pins A3 or A4 is unused, its output force and sense pins should be is approximately 150 kΩ. The gain adjustment trim network connected and the input tied to ground. effectively attenuates the 20 V across the trim potentiometer by a factor of about 1150 to provide a trim range of –5.8 mV to Two outputs of the same voltage polarity may be obtained by +12.0 mV with a resolution of approximately 900 µV/turn connecting both A3 and A4 to the appropriate unbuffered (20-turn potentiometer). The balance adjustment trim network output on Pin 6 or Pin 8. Performance in these dual output attenuates the trim voltage by a factor of about 1250, providing configurations will typically meet data sheet specifications. a trim range of ±8 mV with a resolution of 800 µV/turn. Trimming the AD688 introduces no additional errors over
7 6 4 3
temperature, so precision potentiometers are not required.
A3 1 +10V RB
When balance adjustment is not necessary, Pin 12 should be left
A1 AD688
floating. If gain adjustment is not required, Pin 5 should also be
14 R1 R4
left floating.
R2 A4 15 –10V 20k
Ω
R5 +15V NOISE 2 +15V SUPPLY 1
µ
F REDUCTION R6 R3 0.1
µ
F 7 6 4 3 A2 SYSTEM GROUND 16 0.1
µ
F A3 1 +10V 5 9 10 8 12 11 13 –15V SUPPLY RB AD688
004
A1 SYSTEM 14 GROUND
00815-
R1 R4
Figure 4. +10 V and −10 V Outputs
R2 A4 15 –10V R5 CALIBRATION 2 +15V SUPPLY R6 0.1
µ
F
Generally, the AD688 will meet the requirements of a precision
R3 A2 SYSTEM GROUND
system without additional adjustment. Initial output voltage
16 0.1
µ
F
error of 2 mV and output noise specs of 6 µV p-p allow for
5 9 10 8 12 11 13 –15V SUPPLY
accuracies of 12 to 16 bits. However, in applications where an
SYSTEM
even greater level of accuracy is required, additional calibration
GROUND
may be called for. The provision for trimming has been made
100k
Ω
20T
through the use of the GAIN ADJ and BAL ADJ pins (Pin 5 and
BALANCE ADJUST
Pin 12, respectively).
100k
Ω
20T GAIN ADJUST
The AD688 provides a precision 20 V span with a center tap 00815-005 which is used with the buffer and ground sense amplifiers to Figure 5. Gain and Balance Adjustment with Noise Reduction achieve the ±10 V output configuration. GAIN ADJ and BAL ADJ can be used to trim the magnitude of the 20 V span Rev. B | Page 7 of 16 Document Outline FEATURES GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS THEORY OF OPERATION APPLICATIONS CALIBRATION NOISE PERFORMANCE AND REDUCTION TURN ON TIME TEMPERATURE PERFORMANCE KELVIN CONNECTIONS DYNAMIC PERFORMANCE BRIDGE DRIVER CIRCUIT OUTLINE DIMENSIONS ORDERING GUIDE