Abstract:
A system and method for sampling and holding a signal. The invention includes a novel input circuit for a track and hold circuit comprising a circuit Q 1  for receiving an input signal including an input node, a first output node N 1 , and a path connecting the input and output nodes; a current switching circuit for applying a first current to the node N 1  during a first mode of operation but not during a second mode; and a current source for applying a second current to the node N 1  during both of the first and second modes. The value of the first current is determined such that the total current in the path is constant during the first and second modes. In an illustrative embodiment, the first mode is a track mode and the second mode is a hold mode.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electronics. More specifically, the present invention relates to sample and hold circuits. 
     2. Description of the Related Art 
     Sample and hold circuits (also known as track and hold circuits) are often used in analog-to-digital conversion. A sample and hold circuit (S/H) follows an analog input signal and, at predetermined intervals, holds the input voltage so that it may be converted to a digital value. 
     Most traditional sample and hold circuits, such as that described in U.S. Pat. No. 6,028,459, entitled “TRACK AND HOLD CIRCUIT WITH CLAMP,” operate on the concept of injection of additional current at the base of the input transistor of the sampling gate as the circuit switches from track mode to hold. In track mode, the current on the input transistor is equivalent to a unity current (I). In hold mode, however, the current is effectively doubled in the input transistor as a result of the switching in of a separate, larger current source (with amplitude of 2I), resulting in a delta current of amplitude I in the input transistor. This injection of additional current (or delta current) results in an additional distortion mechanism being introduced within the signal path, and thus deterioration in the spectral purity of the track and hold output. 
     The effect on the degradation of the spectral purity can be addressed. As a result of the switching action, the current in the input transistor changes from 2I to I when the S/H goes from hold to track. This produces a base current step transient that must settle into the input filter thereby degrading the acquisition settling time of the gate. Additionally, the current transient in the input transistor as a result of the discharging of the transistor further degrades the acquisition settling performance. For the track-to-hold transition, the current settling response will affect the hold mode performance at the hold capacitor as a result of the finite isolation of the sampling gate in bold mode: In order to improve both the track mode and hold mode performance, the current in the input transistor should remain constant. 
     U.S. Pat. No. 5,457,418, entitled “TRACK AND HOLD CIRCUIT WITH AN INPUT TRANSISTOR HELD ON DURING HOLD MODE,” discloses a sample and hold circuit in which the current does not double in the input transistor. However, the current is switched from one differential pair to another, creating an unwanted transient similar to that described above due to the finite delays associated with the switching times of the differential pairs. 
     Hence, there is a need in the art for an improved system or method for sampling and holding a signal, which reduces the current transients in the input transistor. 
     SUMMARY OF THE INVENTION 
     The need in the art is addressed by the system and method for sampling and holding a signal of the present invention. The invention includes a novel input circuit for a track and hold circuit comprising a circuit for receiving an input signal including an input node, a first output node N 1 , and a path connecting the input and output nodes; a current switching circuit for applying a first current to the node N 1  during a first mode of operation but not during a second mode; and a current source for applying a second current to the node N 1  during both of the first and second modes. The value of the first current is determined such that the total current in the path is constant during the first and second modes. In an illustrative embodiment, the first mode is a track mode and the second mode is a hold mode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a conventional sample and hold circuit, as described in U.S. Pat. No. 5,583,459. 
     FIG. 2 is a schematic diagram of a conventional sample and hold circuit, as described in U.S. Pat. No. 6,028,459. 
     FIG. 3 is a schematic diagram of a conventional sample and hold circuit, as described in U.S. Pat. No. 5,457,418. 
     FIG. 4 is a diagram of a sample and hold circuit designed in accordance with an illustrative embodiment of the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention. 
     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
     FIG. 1 is a schematic diagram of a conventional sample and hold circuit  10 , as described in U.S. Pat. No. 5,583,459, entitled “SAMPLE HOLD CIRCUIT,” the teachings of which are incorporated herein by reference. The circuit  10  includes a transistor Q 1  having its base connected to an input terminal  1  and its collector connected to a high voltage supply terminal  5 , a pair of series-connected diodes D 1  and D 2  having its cathode of the diode D 1  connected to an emitter of the transistor Q 1 , a constant current source I 2  having its one end connected to an anode of the diode D 2  and its other end connected to the high voltage supply terminal  5 , and a differential circuit  8  including a differential pair composed of a pair of transistors Q 3  and Q 4  and a constant current source I 1  having its one end connected in common to emitters of the transistors Q 3  and Q 4  and its other end connected to a low voltage supply terminal  6 . The transistor Q 3  has its collector connected to the emitter of the transistor Q 1  and its base connected to a sample control input terminal  3 , and the transistor Q 4  has its collector connected to the anode of the diode D 2  and its base connected to a hold control input terminal  4 . 
     The circuit  10  also includes a diode D 4  having its cathode connected to the anode of the diode D 2 , a transistor Q 2  having its base connected to a connection node between the diodes D 2  and D 4  and its collector connected to the high voltage supply terminal  5 , and a differential circuit  9  including a differential pair composed of a pair of transistors Q 5  and Q 6  and a constant current source  13  having its one end connected in common to emitters of the transistors Q 5  and Q 6  and its other end connected to the low voltage supply terminal  6 . The transistor Q 5  has its collector connected to the high voltage supply terminal  5  and its base connected to the hold control input terminal  4 , and the transistor Q 6  has its collector connected to an emitter of the transistor Q 2  and its base connected to the sample control input terminal  3 . Furthermore, the circuit  10  includes a hold capacitor C H  having its one end connected to the emitter of the transistor Q 2  and its other end connected to ground, and a buffer  7  having its input connected to the hold capacitor C H  and its output connected to an output terminal  2  and an anode of the diode D 4 . 
     A shortcoming of this prior art circuit is that in hold mode, the transistor Q 1  shuts off so it is high impedance. This will put a settling problem into the sampling gate input network thereby degrading the overall harmonic distortion of the sampling gate. 
     By shutting off Q 1 , the input of the sampling gate becomes high impedance. This high impedance state is detrimental for several reasons. It takes time for the transistor Q 1  to discharge in the off state in hold mode thereby degrading the hold mode performance of the gate. As well, for the hold-to-track transition, it takes time for Q 1  to charge up and turn on, thereby degrading the acquisition settling time of the gate. An additional problem with this architecture is that Q 1  and the diodes D 1  and D 2  are off in hold mode, creating an unpredictable capacitive divider to the base of the switch, transistor Q 2 , depending on the off-state impedance of these diodes and the on-resistance of diode D 4  in the hold mode. This can cause unacceptably high nonlinear hold mode feed-through at the base of Q 2  that will deposit on the hold cap in hold mode. 
     Other prior art has attempted to overcome these limitations by maintaining current in Q 1  in hold mode. FIG. 2 is a schematic diagram of a conventional sample and hold circuit  12 , as described in U.S. Pat. No. 6,028,459, the teachings of which are incorporated herein by reference. The circuit  12  includes a switching transistor M 2  having a base coupled to an input circuit IS and a collector coupled to a voltage supply V CC . The circuit  12  also includes a differential pair M 5 /M 6  having a collector of M 6  coupled to the base of switching transistor M 2 , and a collector of M 5  coupled to the emitter of the switching transistor M 2 . A hold signal HOLD is applied to the base of transistor M 6  and a track signal TRACK is applied to the base of transistor M 5 . A current source I 3  is connected between the emitters of transistors M 5 , M 6  and circuit ground. A hold capacitor C H  is coupled between circuit ground and the emitter of the switching transistor M 2  by means of a resistor Rc. The circuit  12  further includes a clamping transistor M 4  having a collector coupled to the voltage supply V CC , and an emitter coupled to the base of the switching transistor M 2 . A bias current is applied from a current source I 5  to the base of the clamping transistor M 4 . 
     The circuit  12  also includes an input circuit IS including a transistor M 1 , a current source I 1 , a current source I 2 , and diodes MD 1 , MD 2 . The transistor M 1  has a collector coupled to the voltage supply V CC  and a base coupled to an input terminal  14  for receiving an input voltage signal Vin. A bias current is applied to the emitter of the buffer transistor M 1  from current source I 1 . Diodes MD 1 , MD 2  perform level shifting resulting in the dc voltage at node N 42  being higher than the voltage level at node N 41 , which is the emitter voltage of buffer transistor M 1 . A bias current is applied to diodes MD 1 , MD 2  from current source I 2  which biases the diodes MD 1 , MD 2  on. Typically, the current source I 1  is approximately twice as large as current source I 2 , and current source I 3  is approximately three times larger than current source I 2 . 
     An output circuit OS comprises a transistor M 3  and diodes MD 3  and MD 4  which together buffer the signal held on hold capacitor C H  and level shift the signal up to an output terminal  16 . A bias current is applied from a current source I 4  to the emitter of transistor M 3 . Diodes MD 3  and MD 4  perform level shifting to provide at the output terminal  16  a voltage V OUT , the voltage that is held on hold capacitor C H , and to provide a bias voltage for transistor M 4  when in the hold mode. A bias current is applied to diodes MD 3 , MD 4  from the current source I 5  to bias these diodes MD 3 , MD 4  on. 
     The S/H circuit  12  of FIG. 2 does not shut the input transistor M 1  off in hold mode. For this architecture, there is always a steady state current. However, the current in M 1  changes from track to hold modes. During track, M 1 &#39;s emitter current is I, but during hold, the emitter current is  2 I. This is because I 1  distributes between M 1  and MD 1  and MD 2  in track mode and is only in M 1  in hold mode. Resultantly, the base current in M 1  experiences a transient step for the hold-to-track and track-to-hold transitions. This transient settling will degrade the acquisition settling and hold mode settling of the gate. The reason for this is the transitions produce an impulse of base current at the input of M 1  and this impulse must settle into the impedance seen at the base of M 1 . If the impedance is that of a narrowband filter, the settling time of this error will be very long. 
     FIG. 3 is a schematic diagram of a conventional sample and hold circuit  50 , as described in U.S. Pat. No. 5,457,418, the teachings of which are incorporated herein by reference. The circuit  50  includes a switching device  60   a  comprising a transistor Q 1  having its base connected to an input terminal  42  and its collector connected to a high voltage supply V CC , a pair of series-connected diodes D 1  and D 2  having the cathode of the diode D 1  connected to an emitter of the transistor Q 1 , a constant current source  37  having its one end connected to an anode of the diode D 2  and its other end connected to the voltage supply V CC , and a differential circuit including a differential pair Q 3 , Q 4  and a constant current source  38  having its one end connected in common to the emitters of the transistors Q 3  and Q 4  and its other end connected to a low voltage supply V EE . The transistor Q 3  has its collector connected to the emitter of the transistor Q 1  and its base connected to a sample control input terminal  34 , and the transistor Q 4  has its collector connected to the anode of the diode D 2  and its base connected to a hold control input terminal  36 . 
     The switching device  60   a  also includes series-connected diodes D 3 , D 4 , and D 5  having the cathode of D 5  connected to the anode of the diode D 2  and the anode of D 3  connected to ground, a transistor Q 2  having its base connected to a connection node for diodes D 2  and D 5  and its collector connected to V CC , and three transistors Q 5 , Q 6   a , and Q 6   b  having emitters connected in common to a constant current source  40 , the other end of the current source  40  being connected to V EE . The transistor Q 5  has its collector connected to the emitter of Q 2  and its base connected to the sample control input terminal  34 , the transistor Q 6   a  has its collector connected to the emitter of Q 1  and its base connected to the hold control input terminal  36 , and the transistor Q 6   b  has its collector connected to ground and its base connected to the hold control input terminal  36 . 
     The circuit  50  also includes a hold capacitor  22  having its one end connected to the emitter of the transistor Q 2  and its other end connected to ground. The voltage on the capacitor  22  is provided to output terminal  26  via a buffer  28 . A switch  60   b  and capacitor  24  are connected in parallel to a negative terminal of the buffer  28  and the output terminal  26 . 
     A disadvantage of this circuit  50  is that the transistor D 2  is referenced off of the input signal. This allows the clamp transistor D 2  to be bootstrapped with respect to the signal during the track mode, thereby improving the track mode distortion. The clamp circuit (D 3 , D 4 , D 5 ) is referenced to a hard voltage, independent of the input signal, and thus creates amplitude-dependent distortion in track mode. 
     In the S/H circuit  50  of FIG. 3, an attempt is made to keep the current in the input transistor Q 1  constant in track and hold modes. Q 3  supplies  2 I in track mode, which gets split between Q 1 , and D 1  and D 2 . In hold mode, the current is 0 in D 1  and D 2 , and the current I is provided to Q 1  by Q 6   a . The problem with this approach is the time it takes the transistors in the switch pairs to turn on and off is finite, and during these transitions, the current in Q 1  will be zero, as a result of the finite time it takes for the current to be supplied from Q 6   a  instead of Q 3  and vice versa. Ultimately, if the current in Q 1  could remain fixed in track and hold modes and during the transitions, this would minimize the base current glitch at the input as well as maintain constant current in Q 1  during all transitions. This would help the sampled mode performance of the sampling gate. This is the improvement that the invention presented here provides. 
     FIG. 4 is a diagram of a sample and hold circuit  100  designed in accordance with an illustrative embodiment of the present invention. The circuit  100  includes a switching circuit Q 2  that couples to an input circuit  102  for receiving an input signal Vin and couples to an output circuit  104  for supplying an output signal Vout. It will be appreciated that although the sample and hold circuit  100  is illustrated as a single-ended circuit, it can also be configured as a differential circuit with two S/H circuits  100  to process differential signals. In addition, it will be appreciated that although the S/H circuit  100  is illustrated as comprising npn bipolar transistors, other transistors such as pnp, complementary metal oxide semiconductor (CMOS), n-channel metal oxide semiconductor (NMOS) or p-channel metal oxide semiconductor (PMOS) may be used without departing from the scope of the present teachings. 
     The novel S/H circuit  100  includes a switching transistor Q 2  having a base coupled to an input circuit  102  at a node N 2  and a collector coupled to a high voltage supply Vps. The circuit  100  also includes a differential amplifier  106  comprised of a pair of transistors Q 5  and Q 6  having a collector of Q 5  coupled to the base of switching transistor Q 2 , and a collector of Q 6  coupled to the emitter of the switching transistor Q 2 . A track signal TBHB from a track signal terminal  108  is applied to the base of transistor Q 6  and a hold signal TH from a hold signal terminal  110  is applied to the base of transistor Q 5 . A current source  112  of value I TH  is connected between the emitters of transistors Q 5 , Q 6  and a low voltage supply Vns. 
     A hold capacitor C H  is coupled between circuit ground and the emitter of the switching transistor Q 2  by means of a resistor Rc. The resistor Rc is used to optimize noise and distortions. The circuit  100  further includes a clamping transistor Q 15  having a collector coupled to the voltage supply Vps, and an emitter coupled to the base of the switching transistor Q 2 . A bias current I HP  is applied from a current source  114  to the base of the clamping transistor Q 15 . 
     The output circuit  104  buffers the signal held on the hold capacitor C H  and outputs the signal at an output terminal  116 . In the illustrative embodiment, the output circuit  104  comprises a transistor Q 16  and diodes Q 10  and Q 11 . Since the hold capacitor C H  has a relatively high input impedance, the transistor Q 16  operates as a hold amplifier to buffer the voltage stored on the hold capacitor C H . A bias current I H  is applied from a current source  118  to the emitter of transistor Q 16  to bias the hold amplifier on. The voltage V OUT  at the emitter of transistor Q 16  is output at the output terminal  116 . The diodes Q 10  and Q 11  perform level shifting to provide a bias voltage for transistor Q 15  when in the hold mode. A bias current I HP  is applied to the diodes Q 10  and Q 11  from current source  114  to bias these diodes Q 10  and Q 11  on. 
     The input circuit  102  includes an input transistor Q 1 , current sources  120  and  122 , and a diode Q 14 . Transistor Q 1  operates as a buffer transistor and has a collector coupled to Vps and a base coupled to an input terminal  124  for receiving an input voltage signal Vin. A bias current I is applied to the emitter of buffer transistor Q 1  from a current source  120 . The cathode of the diode Q 14  is connected to the emitter of transistor Q 1 , and the current source  122  has one end connected to the anode of the diode Q 14  and the other end connected to Vps. The diode Q 14  performs level shifting resulting in the dc voltage at node N 2 , between the diode Q 14  and the current source  122 , being higher than the voltage level at node N 1 , which is the emitter voltage of the buffer transistor Q 1 . A bias current of value I is applied to diode Q 14  from the current source  122  which biases the diode Q 14  on. 
     In accordance with the teachings of the present invention, the input circuit  102  further includes a current switching circuit  126  comprising a differential pair of transistors Q 12  and Q 13 , and a current source  128  having its one end connected in common to the emitters of the transistors Q 12  and Q 13  and its other end connected to Vns. The transistor Q 12  has its collector connected to the cathode of diode Q 14  at node N 1  and its base connected to the track signal terminal  108 , and the transistor Q 13  has its collector connected to the anode of the diode Q 14  at node N 2  and its base connected to the hold signal terminal  110 . The current source  128  outputs the same current as the current source  122 . In the illustrative embodiment, the current sources  120 ,  122 , and  128  all output the same current I. 
     The S/H circuit  100  operates in a track mode and a hold mode. The track signal and the hold signal are complementary so transistors Q 12  and Q 13  alternately conduct. When the circuit  100  is in track mode, Q 1  has current I, Q 12  has current I, Q 13  has current 0, Q 14  has current I, and Q 15  has current 0. In hold mode, Q 1  has current I, Q 12  has current 0, Q 13  has current I, Q 14  has current 0, and Q 15  has current I TH . The end result is that by adding the additional differential pair Q 12  and Q 13  in the manner described, the emitter current of Q 1  stays constant whether the device is in track or hold mode. 
     The difference between the S/H circuit  100  of the present invention and the conventional S/H circuit  12  shown in FIG. 2 is the insertion of the differential pair Q 12  and Q 13 , which allows for the reduction in overall current transients by reducing the delta current in Q 1  to zero and avoiding the transient behavior of the prior art circuit  50  of FIG.  3 . This additional differential pair allows for the splitting of the overall DC current source into two sub-components, each with equal magnitude I. When Q 12  is in track mode, it supplies transistor Q 14  with a total emitter current of I as in the traditional circuit. When Q 12  is turned off, the circuit is then switched to hold mode, however the splitting of current allows for the maintenance of constant emitter current on Q 1 , thus reducing the injection of additional transients in the input signal path. 
     The function of the differential pair consisting of Q 12  and Q 13  is to switch the current in the diode Q 14 . By acting as a differential pair for the current switch in Q 14 , the current in Q 1  remains fairly constant during all transitions, thereby reducing the transient at the input network of Q 1  since transients in Q 14 &#39;s emitter current should be somewhat independent of the current in Q 1  and these current transients should be reduced when they are referenced to the input. By keeping the input current fairly constant over time, this reduces any excitations at the input network of the sampling gate, thereby guaranteeing improved acquisition and hold mode settling over the prior art. 
     Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof. 
     It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 
     Accordingly,