Zakładka z wyszukiwarką danych komponentów |
|
CA3306CE Arkusz danych(PDF) 11 Page - Intersil Corporation |
|
CA3306CE Arkusz danych(HTML) 11 Page - Intersil Corporation |
11 / 16 page 4-18 The CA3306 outputs a short (less than 10ns) current spike of up to several mA amplitude (See Typical Performance Curves) at the beginning of the sample phase. (To a lesser extent, a spike also appears at the beginning of auto bal- ance.) The driving source must recover from the spike by the end of the same phase, or a loss of accuracy will result. A locally terminated 50 Ω or 75Ω source is generally suffi- cient to drive the CA3306. If gain is required, a high speed, fast settling operational amplifier, such as the HA-5033, HA-2542, or HA5020 is recommended. Digital Input And Output Interfacing The two chip-enable and the phase-control inputs are stan- dard CMOS units. They should be driven from less than 0.3 x VDD to at least 0.7 x VDD. This can be done from 74HC series CMOS (QMOS), TTL with pull-up resistors, or, if VDD is greater than the logic supply, open collector or open drain drivers plus pull-ups. (See Figure 20.) The clock input is more critical to timing variations, such as φ1 becoming too short, for instance. Pull-up resistors should generally be avoided in favor of active drivers. The clock input may be capacitively coupled, as it has an internal 50k Ω feedback resistor on the first buffer stage, and will seek its own trip point. A clock source of at least 1VP-P is adequate, but extremely non-symmetrical waveforms should be avoided. The output drivers have full rail-to-rail capability. If driving CMOS systems with VDD below the VDD of the CA3306, a CD74HC4050 or CD74HC4049 should be used to step down the voltage. If driving LSTTL systems, no step-down should be necessary, as most LSTTLs will take input swings up to 10V to 15V. Although the output drivers are capable of handling typical data bus loading, the capacitor charging currents will pro- duce local ground disturbances. For this reason, an external bus driver is recommended. Increased Accuracy In most cases the accuracy of the CA3306 should be sufficient without any adjustments. In applications where accuracy is of utmost importance, three adjustments can be made to obtain better accuracy; i.e., offset trim, gain trim, and midpoint trim. Offset Trim In general offset correction can be done in the preamp circuitry by introducing a DC shift to VlN or by the offset trim of the operational amplifier. When this is not possible the VREF- input can be adjusted to produce an offset trim. The theoretical input voltage to produce the first transition is 1/2 LSB. The equation is as follows: VIN (0 to 1 transition) = 1/ 2 LSB = 1/ 2(VREF/64) = VREF/128. If VlN for the first transition is less than the theoretical, then a single-turn 50 Ω pot connected between VREF- and ground will accomplish the adjustment. Set VlN to 1/ 2 LSB and trim the pot until the 0 to 1 transition occurs. If VIN for the first transition is greater than the theoretical, then the 50 Ω pot should be connected between VREF and a negative voltage of about 2 LSBs. The trim procedure is as stated previously. Gain Trim In general the gain trim can also be done in the preamp circuitry by introducing a gain adjustment for the operational amplifier. When this is not possible, then a gain adjustment circuit should be made to adjust the reference voltage. To perform this trim, VlN should be set to the 63 to overflow transition. That voltage is 1/2 LSB less than VREF+ and is calculated as follows: VlN (63 to 64 transition) = VREF - VREF/128 = VREF(127/128). To perform the gain trim, first do the offset trim and then apply the required VlN for the 63 to overtlow transition. Now adjust VREF+ until that transition occurs on the outputs. Midpoint Trim The reference center (RC) is available to the user as the midpoint of the resistor ladder. To trim the midpoint, the offset and gain trims should be done first. The theoretical transition from count 31 to 32 occurs at 311/2 LSBs. That voltage is as follows: VlN (31 to 32 transition) = 31.5 (VREF/64) = VREF(63/128). An adjustable voltage follower can be connected to the RC pin or a 2K pot can be connected between VREF+ and VREF- with the wiper connected to RC. Set VlN to the 31 to 32 transition voltage, then adjust the voltage follower or the pot until the transition occurs on the output bits. The Reference Center point can also be used to create unique transfer functions. The user must remember, however, that there is approximately 120 Ω in series with the RC pin. Applications 7-Bit Resolution To obtain 7-bit resolution, two CA3306s can be wired together. Necessary ingredients include an open-ended lad- der network, an overtlow indicator, three-state outputs, and chip-enabler controls - all of which are available on the CA3306. The first step for connecting a 7-bit circuit is to totem-pole the ladder networks, as illustrated in Figure 17. Since the absolute resistance value of each ladder may vary, external trim of the mid-reference voltage may be required. The overflow output of the lower device now becomes the seventh bit. When it goes high, all counts must come from the upper device. When it goes low, all counts must come from the lower device. This is done simply by connecting the lower overflow signal to the CE1 control of the lower A/D converter and the CE2 control of the upper A/D converter. The three-state outputs of the two devices (bits 1 through 6) are now connected in parallel to complete the circuitry. CA3306, CA3306A, CA3306C |
Podobny numer części - CA3306CE |
|
Podobny opis - CA3306CE |
|
|
Link URL |
Polityka prywatności |
ALLDATASHEET.PL |
Czy Alldatasheet okazała się pomocna? [ DONATE ] |
O Alldatasheet | Reklama | Kontakt | Polityka prywatności | Linki | Lista producentów All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |