Datasheet LTC1609 (Analog Devices) - 9

LTC1609
U U W U APPLICATIO S I FOR ATIO
the end of a conversion, the DAC output balances the VIN LT1363 - 50MHz voltage feedback amplifier. 6.3mA sup- input charge. The SAR contents (a 16-bit data word) that ply current. Good AC/DC specs. represents the VIN are loaded into the 16-bit output shift LT1364/LT1365 - Dual and quad 50MHz voltage feedback register. amplifiers. 6.3mA supply current per amplifier. Good AC/DC specs.
Driving the Analog Inputs
LT1468 - 90MHz, 22V/µs 16-bit accurate amplifier The LTC1609 analog input ranges, along with the nominal input impedances, are shown in Tables 1a and 1b. The LT1469 - Dual LT1468 inputs are overvoltage protected to ±25V. The input im- pedance can get as low as 10kΩ, therefore, it should be
Offset and Gain Adjustments
driven with a low impedance source. Wideband noise The LTC1609 is specified to operate with three unipolar coupling into the input can be minimized by placing a and three bipolar input ranges. Pins R1IN, R2IN and R3IN 1000pF capacitor at the input as shown in Figure 2. An are connected as shown in Tables 1a and 1b for the NPO-type capacitor gives the lowest distortion. Place the different input ranges. The tables also list the nominal capacitor as close to the device input pin as possible. If an input impedance for each range. Table 1c shows the amplifier is to be used to drive the input, care should be output codes for the ideal input voltages of each of the six taken to select an amplifier with adequate accuracy, linear- input ranges. ity and noise for the application. The following list is a summary of the op amps that are suitable for driving the The LTC1609 offset and full-scale errors have been trimmed LTC1609. More detailed information is available in the at the factory with the external resistors shown in Figures Linear Technology data books and LinearViewTM CD-ROM. 3a and 3b. This allows for external adjustment of offset and full scale in applications where absolute accuracy is im- portant. The offset and gain adjustment circuits for the six 200Ω A R1 IN1 IN input ranges are also shown in Figures 3a and 3b. To 1000pF LTC1609 adjust the offset for a bipolar input range, apply an input 100Ω A R2 voltage equal to – 0.5LSB where 1LSB = (+ FS – – FS)/ IN2 IN 1000pF 65536 and change the offset resistor so the output code is changing between 1111 1111 1111 1111 and 0000 0000 A R3 IN3 IN 0000 0000. The gain is trimmed by applying an input 1000pF 1609 F02 voltage of + FS – 1.5LSB and adjusting the gain trim resis- tor until the output code is changing between 0111 1111
Figure 2. Analog Input Filtering
1111 1110 and 0111 1111 1111 1111. In both cases the data is in two’s complement format (SB/BTC = LOW) LT1007 - Low noise precision amplifier. 2.7mA supply To adjust the offset for a unipolar input range, apply an current ±5V to ±15V supplies. Gain bandwidth product input voltage equal to + 0.5LSB where 1LSB = + FS/65536. 8MHz. DC applications. Then adjust the offset trim resistor until the output code LT1097 - Low cost, low power precision amplifier. 300µA changes between 0000 0000 0000 0000 and 0000 0000 supply current. ±5V to ±15V supplies. Gain bandwidth 0000 0001. To adjust the gain, apply an input voltage equal product 0.7MHz. DC applications. to + FS – 1.5LSB and vary the gain trimming resistor until LT1227 - 140MHz video current feedback amplifier. 10mA the output code is changing between 1111 1111 1111 1110 supply current. ±5V to ±15V supplies. Low noise and low and 1111 1111 1111 1111. In the unipolar case, the data distortion. is in straight binary format (SB/BTC = HIGH). Figures 4a and 4b show the transfer characteristics of the LTC1609. LT1360 - 37MHz voltage feedback amplifier. 3.8mA sup- ply current. ±5V to ±15V supplies. Good AC/DC specs. LinearView is a trademark of Linear Technology Corporation. 1609fa 9