Many of today’s data acquisition applications require low-power and/or remote sensing capabilities while occupying a minimum amount of board space. Linear Technology introduces a pin-compatible family of 12-bit and 16-bit A/D converters that meet these requirements. The flagship device of this new family is the LTC1867. It consists of an 8-channel analog input multiplexer (MUX), a high performance and lower-power 16-bit switched capacitor A/D converter, a simple serial I/O, and fits in a small 16-pin narrow SSOP package (5mm × 6mm footprint).
- Sample Rate: 200ksps (LTC1867); 175ksps (LTC1867L)
- 16-Bit No Missing Codes
- 8 Single-Ended or 4 Differential Channels
- SPI/MICROWIRE™ Serial I/O
- On-Board or External Reference
- Low Power Operation: 1.3mA (LTC1867); 0.75mA (LTC1867L)
- Automatic Nap and Sleep Modes
- 16-Pin Narrow SSOP Package
|Input Ranges||0V–4V, ±2V||0V–4V, ±2V||0V–2.5V, ±1.25V||0V–2.5V, ±1.25V|
|No Missing Codes||YES||YES||YES||YES|
Table 1 identifies the differences between the four members of this new family of high performance A/D converters. The members are classified according to supply voltage and resolution. The 16-bit LTC1867 and the 12-bit LTC1863 operate on a single 5V supply while sampling at 200ksps. The LTC1867L and the LTC1863L operate on a reduced 3V supply with the same features of the LTC1867 and the LTC1863, respectively. The LTC1867L and LTC1863L key specifications are guaranteed for 2.7V operation. In addition, all four parts provide an automatic Nap mode, Sleep mode, unipolar and bipolar operation, and an internal bandgap reference. Figure 1 shows a block diagram for the LTC1867/LTC1863.
The eight-channel analog input multiplexer can be selected either in 4 differential pairs, 8 single-ended, 7 single-ended channels versus COMMON pin (pin 8, CH7/COM acts as COMMON MINUS) or combinations thereof. These configurations are set up by a 7-bit input word defined in Table 2 through the SDI/SCK serial port. The channel-to-channel matching for offset and gain error are excellent. For the LTC1867, the offset error match and gain error match are both specified as ±2LSB (max). The crosstalk between channels is typically better than 110dB. Figure 2 illustrates the flexibility of the 8-channel MUX.
|1. SD = SINGLE/DIFFERENTIAL BIT
|2. OS = ODD/SIGN BIT
|3. S1 = ADDRESS SELECT BIT 1
|4. S0 = ADDRESS SELECT BIT 0
|5. COM = CH7/COM CONFIGURATION BIT
|6. UNI = UNIPOLAR/BIPOLAR BIT
|7. SLP = SLEEP MODE BIT
Unipolar and Bipolar Mode
These A/D converters can sample the difference of positive input (+VIN) and negative input (–VIN) at the same instant either in unipolar or bipolar modes depending on UNI bit of the Input Word. For the LTC1867 and LTC1863, the input ranges (i.e. +VIN minus –VIN) are 0V to 4V in unipolar mode and ±2V in bipolar mode. For example, the +VIN can swing from 0V to 4V if –VIN is tied to Ground in unipolar mode, and the +VIN can swing from 0.5V to 4.5V if –VIN is tied to 2.5V in bipolar mode.
Figure 3 depicts the unipolar and bipolar operation for the LTC1867. On the upper half of Figure 3, the LTC1867 samples and digitizes a sine-wave on Channel 1 (CH1) that swings from 0V to 4V and measures a DC voltage of 2.5V on Channel 0 (CH0), both in unipolar mode. On the lower half of Figure 3, the ADC performs a bipolar operation with CH0 versus CH1, and vice versa. Under these conditions the differential input signals applied to the ADC exceed the bipolar input range of ±2V. For example, in the case of CH0 minus CH1 the resulting digitized input range will be –1.5V to 2.0V.
In both unipolar and bipolar operation, +VIN and –VIN are sampled simultaneously; so common mode noise on both inputs is rejected by the truly differential inputs. In the unipolar mode, the +VIN range of the LTC1867 is from 0V to VDD and the –VIN range is from 0V to VDD/2. Some competing devices only allow the minus input to reach a few hundred millivolts to one volt above ground. By comparison the LTC1867 offers about twice the range on the minus input. This makes the LTC1867 a great choice for remote applications where large common mode voltages can be present. In the bipolar mode, both +VIN and –VIN can accept signals from 0V to VDD.
Simple Serial I/O Allows Easy Isolated or Remote Applications
The serial I/O employed by the LTC1867 and its other family members is compatible with the industry standard SPI/MICROWIRE interface. The shift clock (SCK) synchronizes the data transfer with each bit being transmitted on the falling SCK edge and captured on the rising SCK edge in both the transmitting and receiving systems. After the conversion is complete, the input word is shifted into the ADC through the SDI/SCK ports, and at the same time, data bits are released through the SDO/SCK ports. The A/D converter starts to acquire the analog input signals after reading in the 7-bit Input Word. These ADCs have an internally trimmed conversion clock which allows the sampling frequency to approach DC without affecting the conversion results. The 4-wire interface allows the LTC1867 and its siblings to fit well with isolated or remotely located applications.
Outstanding DC and AC Performance
Operating from a 5V supply and sampling up to 200ksps, the LTC1867 delivers 16-bit, no missing codes performance with an accurate INL specification of ±2LSB(max) in unipolar mode and ±2.5LSB(max) in bipolar mode. Typical INL and DNL plots for LTC1867 versus output code are shown in Figures 4 and 5. This performance is achieved with capacitor matching which is very stable over time and temperature.
Along with outstanding DC performance, the LTC1867 also has very good AC performance. The signal-to-noise ratio (SNR) is typically 89dB with an input range of 4V and improves to 90.5dB when an external reference voltage of 5V is applied to the REFCOMP pin (tie VREF pin to 0V to turn off internal reference buffer).
This family has an on-chip, temperature compensated, curvature corrected, bandgap reference that is factory trimmed to 2.5V for the LTC1867 and the LTC1863, and 1.25V for the LTC1867L and the LTC1863L. The reference is internally connected to a reference amplifier and is available at VREF (Pin 10). A 6kΩ resistor in the LTC1867 and the LTC1863 (3kΩ resistor for the LTC1867L and the LTC1863L) is in series with the output so that it can be easily over-driven by an external reference if better drift and/or accuracy are required as shown in Figure 6. The reference amplifier gains the VREF voltage by 1.638 to 4.096V at REFCOMP (Pin 9). This reference amplifier compensation pin, REFCOMP, must be bypassed with a 10μF ceramic or tantalum in parallel with a 0.1μF ceramic for best noise performance.
Low Power Improves Battery Life
The LTC1867 and LTC1863 consume only 1.3mA at a sampling rate of 200ksps. As the sampling frequency is reduced, the converters use even less supply current with the automatic Nap feature. For example, the parts draw only 760μA and 200μA at sampling frequencies of 100ksps and 10ksps, respectively.
Automatic Nap mode is active when the CS/CONV pulse width is longer than the conversion time of the A/D converter. The part goes to Nap mode automatically right after a conversion is completed and remains powered down (the ADC draws 150μA in Nap mode) as long as the CS/CONV stays HIGH after conversion. The internal reference, however, is still active and provides a 2.5V output. In this way, the LTC1867/LTC1863 requires no additional wake up time before the next conversion is started. Figure 7 shows how the supply current is greatly reduced as the sample rate is decreased when using this feature.
The ADCs can also go into the Sleep mode during long inactive periods. In sleep mode the internal reference is also powered down, thus reducing the draw to leakage currents of less than 1μA. The wake up time out of the sleep mode is determined by how fast the reference bypass capacitors can be charged. The wake up time can be estimated with the values of bypass capacitor on VREF and the on-chip resistor between the internal reference and VREF pin. For the 16-bit LTC1867L, the wake up time can be estimated as:
(resistor value) • (bypass capacitor value) • (number of time constant needed to settle to 16-bit accuracy)
or (3k • 2.2μF • 11).
Typically with bypass capacitors of 2.2μF and 10μF on the VREF and REFCOMP pins, this takes about 80ms for LTC1867L. However, if an external reference is used, the wake up time is less than 10ms.
When the CS/CONV pulse is shorter than the conversion time, the ADCs stay powered up and the Automatic Nap mode is not activated. In this configuration, the digital output, SDO, becomes active after the conversion is completed. Figures 8 and 9 show the timing diagrams for the two cases described.
The LTC1867 family packs an 8-channel analog input multiplexer, low-power A/D converter, serial I/O, and an internal reference in a narrow 16-pin SSOP package. With outstanding DC and AC performance, and equipped with the automatic Nap and Sleep modes for power reduction, these complete A/D converters can be used in many space-sensitive as well as low power applications.